Your ad hoc percentage is wildly exaggerated. And what do you mean by multiclassing? Do dips count? Prestige classes? Generally, except for full casting classes (and in this instance, the bard doesn't count, as losing some casting doesn't hurt as much for him), if a second class will give an ability you want, or adds an already high stat to something like saves or AC, it's in no way underpowered to take some levels in it.
Yes, I include PrC and dips. I come from playing Living Greyhawk as my primary D&D with 1 or 2 home games ongoing throughout the years.
However, in LG, the encounters you went against were DEADLY or a regular basis. We were use to the difference in winning or losing being a couple of points in either direction(either to AC, hitpoints, damage, attack bonus, healing or whatever).
So, you ask if it was unplayable if a fighter 10 took a level of wizard? Probably not, but the party is a lot weaker. 1st level spells won't help us at all playing APL 12 adventures. Is the same fighter who is 10 fighter 5 wizard unplayable? Yep. He lost so much BAB that he will miss too often to be a good fighter. He'll have too few hitpoints to protect the party and his AC is either way too low or he has way too high an arcane spell failure to even use his spells.
There are actually very few multiclass choices that are viable. The general rule in our group was: If you are a spellcaster and can't cast spells at least one level lower than a pure spellcaster of your level, you are not viable. If you are a fighter type and you ever lose more than 3 points of BAB in 20 levels, you are not viable. If you are any levels of Bard, you are not viable(ok, that's mostly a joke...but semi-serious).
That means that the combinations that come up with viable combos are far and few between. The thing is the tradeoff in multiclassing. Almost every multiclass out of your primary class was bad unless it was a dip into a class that made you better at what you do. You could justify a 1 level dip into Barbarian if you were a fighter because it gave you more hitpoints, the ablitiy to rage, and still gave you the BAB you'd get for going up a level in fighter. In fact, it mostly just made you better.
One level of wizard might be able to help, if you had the actions to waste on spells. Most of the time the extra hitpoints and BAB, and possibly feat helped you more at what you were actually doing: Attacking enemies with your weapon. The thing is, a 1d4+1 damage magic missile does nothing to a high level enemy. You might be able to find a spell that made up for what you lose, but that's my point. If there are 2 spells that make up for the loss, and 50 more 1st level spells in the game then everyone who takes the 1 level of Wizard who doesn't take those 2 spells is hurting themselves badly. Since all of those are "multiclass options", 90% of all Fighter 19/Wizard 1 builds are bad.
If you carry that across all classes, you get that there are HUGE numbers of multiclass builds that are worse than their single class counterparts designed to play a similar role. Worse yet, the reverse is true. If you were able to take 1 drops creatively based on which class gave you more than your normal class, then you could be way more powerful than a single classed character.
Back on the original topic, this is why there could be a lot of loner characters who worked just fine in 3e. Power varied so much in 3e that a group who was used to multiclassing as the norm often became fairly below average power. The DMs in these games compensate by using lower CR creatures. If you are using mostly CR 8 or 9 creatures against your APL 10 group, someone who was really good at powergaming could easily abuse multiclassing to be the most powerful person in the party, and able to consistently one or 2 shot enemies.
And when you're that much more powerful than the rest of the party, it lets you do the loner thing really well. You can ignore the tactics of the rest of the party, because whatever you attack dies in one or two shots. If the party disagrees with you, you can threaten to leave and they beg you to stay because of how powerful you are.
This is, of course, assuming the DM doesn't get frustrated at all his monsters dying in 1 hit and increase the power of the enemies accordingly....and kill off the rest of the party.