The thing is (and this is one of the things they've said they're fixin') that each class should be built that, unless your character concept desires multiclassing, you won't WANT to multiclass just to gain some mechanical advantage. And that if your character concept desires multiclassing, there should be no limit on it.
So, no, not every wizard would take a level in monk for AC and save boost, because multiclassing as a spellcaster *cripples* your high level effectiveness. If you're fighting CR 5-6 critters and don't have Fireball or Lightning Bolt, your party's going to have a harder time of things. Basically, another level of Wizard should be better than level one Monk powers in the same way that another level of Wizard should be better than level one Ranger powers, etc. Right now, this seems true -- no spellcaster in any of the games I've run has been even half tempted to multiclass as a monk because they'd loose all the neat spells...I mean, Mage Armor is as good as Wis Bonus to AC, Shocking Grasp is better than an unarmed strike, and Charm Person is better than stunning fist.
And I think that the 'The DM can't take these away!' is mostly a moot point, because (IMHO) any DM worth his salt doesn't create challenges by taking away the player's abilities, they do it by playing to the strength of the characters. So maybe there will be ONE adventure out of an entire campaign where the PC's find themselves stripped of equipment just to let the Monk shine a bit. But if there's no Monk in the party, that adventure probably won't exist, because it will just universally torque off the players.
They've said they're going to alter the front-loaded-ness of some of the classes (like Monk or Paladin), so the early abilities of those classes are also mostly a moot argument (and wouldn't prevent someone from getting it besides).
So, then, the argument is that the high level powers of the classes are more powerful than the high level powers of other classes? Well, (a) nothing now stops someone from getting to a high level and then switching classes just for the high level benefit, and (b) if they have to get to a high level for their 'reward' anyway, then they're going to be craptastic in any other class they choose. Also, I have a hard time *believing* that argument, since that would make the monk or paladin a more powerful class than others...and that would throw the entire CR/EL system outta whack...we'd have entries like CR 4, unless you're party has two or more monks in which case CR 3, since if they're more powerful, they're going to be able to handle harder challenges more easily than other characters.
So, I guess, from my experience:
1 -- Even opening up the mutliclassing won't result in people sacrificing high-level powers for low-level front-loaded powers.
2 -- The front-loaded quality of many classes is being rectified in the revision.
3 -- Having an ability that the DM can't take away doesn't make it more powerful, since DM's shouldn't be making challenges that depend upon stripping the PC's of their powers. In which case, they're as easy to take away as anyone else (New Spell: No Ki Zone!)
4 -- The higher level powers of the class being 'more powerful' than respective powers doesn't seem to be realistic, and the fact that they have to be high level (and thus invest a lot of time into the class) to get the benefit. Also, the current restrictions don't forbid themf rom getting those high-level powers, and then switching to another class anyway.
So, I guess, there seems to be ABSOLUTELY NO mechanical reason to preserve these mutliclassing restrictions. And if there is a flavor reason, then it should be relegated to an optional rule, much like how racial class restrictions are an optional rule (since that's also a flavor consideration).
