Here's the way I see the breakdown anyway:
The primal fighter: Barbarian
The skilled fighter: Fighter
The unarmed fighter: Monk
The cheap-shot fighter: Rogue
The divine caster: Cleric
The primal caster: Druid
The arcane caster: Wizard
The mystic caster: Psion
IMO rangers have such a bad outlook because they are too many things which are sort of subsets of the other classes. Paladins are really cleric/fighters in many way, but instead of as much spell power, they have other divine gifts.
Sorcerers are arcane casters who simply have a different method of getting their spells. And instead of as many, they learn to warp them.
Warlocks are also just "out there" like the other classes. They "sort of" have a niche, but instead of warping spells like a Sorcerer, they are more akin to Paladins in that their magic has given them other "gifts".
Bards are basically rogues with a whole heap of magic thrown on top (too much, IMO, but whatever...).
How unique the Psion could become depends entirely how it is developed, but so far every attempt to make one has fallen flat because we can't arrive at a large enough consensus about it.
The warlord is in a similar boat, but could easily be a subclass of Fighter IMO and doesn't warrant a full class. ANY class can lead others, really, and that is another issue I have with Warlord.
I know a lot of people don't like prestige classes, but I think that is a great way to develop a character and then branch them off to really fill a particular need or concept.
Anyway, my argument why we don't need more classes is because IME every concept can be realized with even less than the classes we have now. If the class doesn't completely do it alone, that is where subclasses should come in IMO.