*Slight, quick edit, since I sort of ramble in making my point: I'd prefer D&D to have 7-12 distinct classes representing a variety of archetypes well, and each reasonably customizable to produce whatever sub-archetypes may be fitting, within the greater niche of the class.*
I don't think D&D would really be D&D anymore if certain things were taken out of it, including classes. I think the same would happen if D&D's classes were cut down to just a paltry 2-5 overly-generic-and-cookie-cutter classes. Multiclassing is part of the system for a reason, and the fact that it may be necessary for certain character concepts isn't a bad thing, it's a feature. Want a ninja? Multiclass fighter-rogue then, ninjas never got their bad-assed-ness from being lazy bums training only on weekends and only after reaching adulthood, they had to work at it, dangit. In 3E, just use the apprentice-level character rules in the DMG, and wham, you've got a 1st-level PC who's a fighter/rogue on their way to learning the basics of ninjitsu. You're not a real ninja anyway until you're at least 3rd-level or higher, because only inexperienced punks with insignificant amounts of training are still 1st or 2nd level.
As far as I can tell, discern, or comprehend, some of the things that just make D&D what it is are classes, levels, experience points, races, proficiencies/skills, an arcane/divine magic divide, and at-least-vaguely-Vancian magic. Think about it; if you took half of these away, you'd basically be playing any other system altogether, except you'd be rolling a d20 instead of several d6s for most stuff during play. If you ditch classes or levels, you may as well be playing GURPS, HERO, Shadowrun, or something else of the sort, because it's obvious that the kind of system you prefer for representing your fantasy campaigns is not D&D. Not that it's a bad thing, I love Shadowrun myself, but the way that classes, multiclassing, and levels work in D&D are a large part of what makes it D&D, mechanically, in terms of how it represents the fantasy world. It strengthens the archetypes and gives discernable roles for players, and it makes them meaningful, it gives them a place in the campaign world and in turn defines the style of the game. The way that Wizards are defined in D&D makes it different from many other settings; the way that Monks, Rogues, Bards, Clerics, and Druids are defined; these strongly-defined archetypes in D&D are a significant part of what makes a D&D setting still look, feel, and play distinctly like D&D, rather than RIFTS, Shadowrun, GURPS, MERP, WHFRP, or whatever.
Classes and levels provide measuring sticks, baselines, and archetypes for use in play, and they make DMing/GMing less of a chore, generally. They also help in building a character, except when going for some complex concept that really requires a high-level-or-equivalent character to pull off in any game. Classes are more distinctive and defining than merely lumping a pile of feats/abilities/skills/whatever from a classless system and calling them "the common warrior" or whatnot. We wouldn't have the Barbarian, the Monk, the Paladin, or the Bard. Without a well-defined, reasonably-large class system, such concepts would either be horribly crippled compared to more-munchkinized combinations of stats from the master list, or they'd be just plain impossible to duplicate. Without a paladin's strict set of codes and abilities, they'd be rather more powerful; if their less-frequently-useful abilities were taken away and replaced with more potent, more efficient upgrades to their commonly-used abilities, they'd be too munchkin. If a monk could forego all the mystic mumbo jumbo and just put all their points into unarmed accuracy and damage, they'd be the one-hit-wonder, wantonly killing every enemy in sight with their uber-death-punch-of-ultimate-doom, because really, the worth of their versatile and numerous powers would equate to such a death-touch if combined and spent more efficiently from a pool of character points. Instead, as a strictly-defined class, the Monk in play is actually terribly mediocre and sub-par in both combat and out of it, generally (I've played tons of Monks in 3.x D&D, much to my disappointment), but at least has enough versatility and cool little powers to be interesting and kinda fun sometimes, at least, even if it's still a second-rate combatant relegated to helping the Rogue flank, and at the same time a second-rate spy or diplomat, lowly sidekick to the Bard or Rogue in scouting or negotiations.
For such reasons, I don't care at all for the Unearthed Arcana warrior/expert/spellcaster three-class option, it's horribly generic in my mind and doesn't allow any meaningful distinguishing between characters. I don't like D20 Modern's over-simplified, ability-score-based classes either, they just seem wonky to me, and don't really represent any particular concept; what the heck is the real difference between a Smart Hero and a Dedicated Hero, in concept and definition? Nothing, that's what. Warrior A is much like Warrior B, and plays almost exactly the same, and it doesn't matter if Warrior A only ever studied swordsmanship and Warrior B only ever fought in bar-room brawls without any formal training let alone weapon training. Warriors A and B in such overly-simple systems just don't make sense in my suspension of disbelief, when Swordsman Bob (aka Warrior A) is disarmed by Brawler Joe (aka Warrior B), who then takes Bob's sword and proceeds to hack him to pieces with it, despite the fact that Bob knows best how to use and dodge sword attacks, while Joe has never so much as picked up a pointy stick in his entire life, let alone swung a 4-pound double-edged blade 4-1/2 feet in length. And so on and so forth. There needs to be a decent amount of differentiation between characters of the same general sort (not everyone who's a Fighter ought to be a master with any sword, hammer, bow, polearm, or whatnot that they happen to pick up, nor should they all be tough and hard-hitting and devastatingly accurate). When a slow, greatclub-wielding, heavily-armored warrior of great experience faces a quick, nimble fencer of mediocre experience, the club-wielder should not always hit the fencer just because he's more experienced, as his slow attacks and cumbersome armor should be easy enough for even a mere halfway-decent fencer to avoid, while the fencer should have a reasonably easy time hitting the slow, armored brute. Just because the brute's an 8th-level Fighter and the fencer's a 3rd-level Fighter doesn't mean the plodding, clumsy brute should be so deadly-accurate with his heavy club.
Classes and levels, when there aren't just a pitifully small number of them, help to differentiate characters sometimes, and keep munchkins in check when other folks just want to have a little fun in the game without spending hours maximizing their choices of stats and abilities from some master list. It's no fun when the munchkin's powerhouse wipes out every villain before fun Mr. Bard can even draw his blade, just because the munchkin's character is ungodly fast and assassin-like in his efficiency. Or when the munchkin's necromancer kills the Big Bad with one spell in the first round of combat just because he took the obscene amount of time to maximize the character's ability with that spell to the point where it was impossible for any similarly-leveled enemy to resist. Because if the DM/GM goes to the excessive amount of effort needed to compensate for the munchkin and give the other PCs something to do, it'll just mean the other PCs getting slaughtered if the munchkin decides after, *gasp*, actually missing/failing-to-insta-kill with his first attack against the enemy, that maybe he'll just leave that baddie alone and run off to find whatever "trick" the DM "must've" left in the area or back at the town "for the munchkin to inevitably find" and beat the "invincible" enemy with. Etc. Dealing with/compensating for munchkins is too much work and too frustrating for players and DMs/GMs alike. Classless systems are generally much more viable for munchkins to ruin a game with.
I've just noticed a lot of very-obviously overpowered, munchkinny junk in classless systems to believe that my casually-constructed, supposed-to-just-be-fun character in such a game will have any chance of ever doing anything meaningful, other than being an amusing, pathetic, useless sidekick to the godly munchkin character who solves everything through his rules-fu and munchkin-senses. At least with a classless system there's more control, balance, and guidance. I could ban a class that's overpowered; but would I really want to ban a feat/talent/ability/whatever in a classless system when that ability is really only overpowered in certain munchkinny combinations, and otherwise would be very neat and flavorful for other, less grossly-optimized characters? Banning a particular class or feat, altogether, before the campaign, is doable and usually won't upset anyone in particular unless they were really, really itching to play a character of that type. I could always run a separate game later that allows that class though, and is designed to be more hack-and-slash or powergamey or whatever to work well with it, but it isn't so simple or easy, typically, with a classless system and a classless, freeform character design.
If I just ban a character individually that I think is too munchkin to let anyone else have fun, I'll be branded a jerk and a favoritist. I've had folks walk out on my games just because, on the day the game started, I looked over their completed character sheet and said "I'm sorry, but I don't really think this character/feat/whatever is fair in my game" or "Sorry but that kind of character doesn't fit my campaign setting/campaign region/whatever" or "How did you come up with these stats? I don't think that's really fair to the other players, I already told them weeks ago what rolling methods/point-buy-totals/whatever to use" or anything else vaguely of that nature.
And now I've made the post too long since I descended into ranting. Ah well. The first paragraph or two probably get my point across more succinctly and rationally.... Needless to say, I am strongly in favor of D&D's future incarnations remaining distinctly D&D-ish and not just pitiful, marginally-different reflections of other roleplaying systems that are less distinct and traditional.
