Pemerton said:
Judging from what Mearls said about the Beholder in his monster makeover column, and also drawing inferences from the Dragon combat example posted in the recent Design and Development column, the logic of a Beholder will be that it is the functional equivalent, in combat, of 5 more typical monsters/NPCs. This requires that the Beholder have many actions, and certainly more than a PC can have.
To allow a PC to be the functional equivalent of 5 characters would defeat the purpose of the mooted monster design principles,
That's exactly why this wouldn't work in 3e, but it's shortsighted. There's no reason a Beholder HAS to be as powerful as 5 other monsters. The new encounter design is 4 monsters/4 PC's. The Beholder can be as powerful as ONE PC, and not loose one bit of it's beholder-osity.
This is just one example to show that monsters can vary in parameters (in this case, the number of typical characters to which they are functionally equivalent) which should not be allowed to vary for PCs, if the desired style of game play is to be preserved.
It may be, of course, that one doesn't care for that style of play, in which case one will not care for (at least this aspect of) 4e.
There's no reason for a Beholder to be as powerful as 5 other PC's in the new edition, so your reason falls flat: if the beholder is designed to be one monster among 4 in an encounter, it can be designed to be one monster among 4 in a PC party of the same level.
If a Mind Flayer or Drow encountered in the field becomes an ally and joins the party, then its numbers will not change - they are already known. But there will be no quick-and-easy way of assigning a PC level to that ally.
If a player wants to build a PC Mind Flayer or Drow from the get-go, then there is no guarantee that such a character will be able to built, using the level/skill/feat/talent-tree rules, in such a way as to deliver something numerically equivalent to a Mind Flayer or Drow that one might meet in the field.
That's a problem, because it shouldn't be hard to do. The designers have an opportunity to address that concern NOW that they won't have again until 5e.
Does this mean we are having to squint at pseudo-Drow? I don't think so, because (as I understand the direction that 4e is taking) there is no correct answer to the question "What are the true numbers for a Mind Flayer, or a Drow?" The numbers, for monsters/NPCs, will vary depending on the role and challenge that the GM wants the creature to pose. So your PC Drow/Mind Flayer, with its own PC-build derived numbers, is just as much a genuine creature as any of those that one would encounter in the field.
If the drow fighter in the party and the drow fighter the party is facing have dissimilar and incompatible abilities, my realism is curb-stomped. If the abilities are similar and compatible, there should be no reason the PC can't get it if the NPC can.
At the moment, if the hungry PCs are wandering lost through a forest, you as a GM have to determine:
*Do they meet anyone?
*If so, is it a centaur?
*If so, is it a centaur Barbarian, Druid or Ranger who might be able to help them survive?
In 4e you have to answer the same questions. So there is no more or less making up of stuff. If you don't want to decide, and instead prefer to use random tables that reflect the in-game likelihood of encountering creatures, including centaur Rangers of a given skill level, you will be able to use the same tables in 4e.
In 3e, the answers to those questions are a few die rolls away: do they meet anyone? Roll encounter chance. Is it a centaur? Roll on the encounter table. Is it exceptional? Not if I don't already have the stats.

Can it help them survive? It has a Wisdom bonus, and it has listed skills, let's roll a check for it.
I don't really have to make anything up at the table there (unless you count adding class levels to the centaur, which, again, I wouldn't do unless I had set it up beforehand). I just have to let my brain leap from logical point to logical point and let random tables fill in the gaps.
In 4e, if I have to arbitrarily decide how well a given centaur can find edible food for the party, I will pretty much scrap the monster manual as "not designed for my uses."
In 3e you can only tell how good a tutor the Centaur will be once you have decided what HD the Centaur has (and therefore what extra skill points and feats it may have acquired through advancement), what class levels it has (and therefore what extra skill points, feats and class abilities it may have acquired) and what magic items it has. The GM has to make those decisions, and together they determine the outcome to the question posed.
There's a centaur in the MM, y'know. And treasure generation methods to find out what kind of stuff it might have. All the descisions there are already made, a few die rolls away at most, and unless I do some extra work for a specific purpose, there's nothing left for me to invent about it's stats. Which is perfect, because then I can concentrate on running the encounter and not pondering the mysteries of centaur skill points.
In 4e the GM will have to make a different set of decisions: What numerical bonuses do I want this creature to have? Once those bonuses are assigned, the question of tutoring utility will be answered. If you don't want to choose, use the same random determination process you are using at present.
So what am I paying these guys for, if not stats? Fluff? Artwork? Why should I have to decide what bonuses the creature has when, presumably, the purpose of the MM is to have that work already done for me?
And my method of "random determination" isn't at all. I look at the stat block, and I say "Well, looks like the typical centaur isn't that good a teacher. Maybe Chiron is an Expert with some ranks in Profession (Teacher). Looks like I may have to do a little pre-prep for this game." Or I say "Well, they're pretty wise, so maybe they can give the PC's some aid."
I have a starting point and I divert from it for variety. If 4e doesn't give me a starting point, 4e's monster design sucks for my purposes.
Sure. But what precludes you extrapolating the Survival skill of a Centaur from your knowledge of Centaurs in your gameworld? Or your knowledge of this particular Centaur.
Again, I'm an improv-heavy DM. I don't spend much, if any time pondering the mysteries of centaur survival outside of the times it comes up at the table, and when it comes up at the table, I want to have an answer RIGHT THERE, so the game can keep plugging along. That particular Centaur may not have appeared before I randomly rolled him on an encounter table, or before I had a character in a nearby town mention him (which I didn't anticipate doing) or before the party druid, just five minutes ago, mentioned how he would like to train under a truly wild creature, or before I chose some arbitrary creature of the appropriate CR...in fact, Centaurs may not have existed in the world at all before the party encounters them.
So my knowledge of centaurs in the world, and of that particular centaur, may not be any deeper than the PC's knowledge at the moment, and may, in fact, be more shallow. I like DMing that way -- it keeps me on my toes and keeps nearly all of my work directly useful. But in order to DM that way, I need a very solid baseline that I can pull from, to keep things fair and to ensure verisimilitude. I need monsters that are more than just XP gristle. I need monsters that are part and parcel of the world I throw them into, down to whatever probably-irrelevant detail that I need to render them complete.
Some of those roles are "functional", in terms of the sort of game mechanical sub-system they relate to: "brute" and "archer" are both combat roles, for example, while "wilderness protector" suggests a role in either survival or social challenges, and "potential ally" and "trainer of heroies" both suggest a social challenge role.
Others of those roles are broadly "flavour" roles which can overlap with any given functional role: "spirit of hedonism", "underworld terror", "monstrous brute from a distant land", etc.
There's no reason to suppose that the game won't support centaurs filling all those various functional roles, and nor to suppose that it will preclude you from using one or more of the flavour roles.
If it doesn't tell me their Diplomacy bonus, it doesn't support me using it as a social challenge. If it doesn't tell me their Survival bonus, it doesn't support me using it as an ally to the PC's. If it's combat abilities would be unbalancing in the hands of a player, it doesn't support me using it as a consistent party member. If it doesn't give me a solid, stable baseline, it doesn't support me departing from that.
If things are excepted because they're "not relevant," and if monsters are designed solely for combat, without considering their game-world and player-character usefulness, there's no reason to believe that Wizards really wants me to play 4e, because they're not going to be designing for what I need at a game table.