OP, I think you've misunderstood the kind of balance that I think the designers (and a lot of us here on the boards) are referring to when we yammer on about balance. See, to my mind, there are two kinds of balance - competitive, and cooperative.
Competitive balance requires that all classes adhere to certain norms, and demands that they all strive to have a similar level of overall power. This is ... not an undesirable goal, but only really an essential one in multiplayer games. TF2, Starcraft, and League of Legend, for instance, all try to have competitive balance. The issue with competitive balance is the one you raise - homogeneity. Variety requires that balance be asymmetric, and asymmetric balance is really ****ing difficult to achieve. Homogeneity is a solution, or at least a stopgap, to this difficulty.
Unfortunately, when we talk about balance, people advocating both for and against it often adopt the position that all balance is competitive balance, and therefore assert either that (a) games should not be balanced, or (b) options in games should be mostly homogenous in the interests of balance. There are problems with both of these two positions, especially when it comes to games like PoE, where cooperative balance is of crucial importance.
Cooperative balance is about allowing every option (classes, in the case of PoE) to fill a distinctive role, having unique things that they do best. Ideally, those unique things should be no more or less valuable than those of other classes, but that's less important. What's very important is avoiding traps in character creation and customization, such that one option is simply like another, but worse. Balanced design is about creating a variety of distinctive choices, none of which are wrong by virtue of not treading on one another's toes.
This is a place where the IE games were really a mixed bag, actually, if you were playing in an optimized way. For example: In BG2, unkitted, Stalker, and Beastmaster rangers in are flat-out worse than Fighter/Thieves (or Fighter/Mage/Thieves, if you really want to insist that ranger spellcasting is significant, which it isn't). There is nothing that any of those rangers can do that an F/T (or F/M/T) can't do better, and many things an F/T (F/M/T) can do that the Ranger is incapable of. One class is strictly superior to the other - such that if I want to play a vanilla ranger in BG2, I'm better off playing an F/T and just calling it a ranger. There are lots of similar issues - by mid-levels, for instance, a buffed-up cleric is better at fighting than a fighter; Berserkers are largely just barbarians with better equipment options; Cleric/Rangers are druids with better THAC0, more and better spells, and no worthless shapeshifting abilities; so on and so forth; etc (who, me, bitter?).
PoE, in trying to be conscious of balance, has presented us with classes that actually are very different from one another. Ciphers, Rangers, and Chanters are weird and interesting. Priests and Druids show tremendous promise. Paladins and Fighters feel like different classes 90% of the time, instead of the 10% common to the IE games. These are just a couple of examples - for all of my concerns about PoE's beta, class diversity is absolutely not among them. And I'm confident that it will be maintained, and probably even advanced, as development goes forward.
tl;dr Balance is good, and PoE's take on it does not restrict class diversity, but rather produces it.