Valiant Heart:
While a dm appointed prc has certain powers which can possibly be emulated by clever spell research, I would not say that makes prcs redundant.
Some concepts go beyond the core classes and what the core classes are capable of. Dragon Disciple is a good example. You begin to take on characteristics of a dragon, the "cost" is levels. If you made the "cost" feats, it would be too easy, if the "cost" was spell research and exp usage (upon completion of the spell) its overly limited to wizards researching spells AND would be very difficult to balance properly. How many exp = wings and greater hit die?
The far easier answer is to create an alternate class (we'll call it a prestige class) which gives you virtual "dragon metamorphosis" levels. Taking these levels doesn't let you do any of the things your class normally does, except resist damage, make saving throws, and hit things, and we'll make a unique skill list.
Is it possible to have emulated this 10-level prc with feats, spell research, magic items, or divine intervention? Yeah. Which way is easier?
Someone quoted Monte Cook and his perception of a prestige class. I say his perception because even if the idea was wholly his from the beginning, it has grown beyond his initial concept. Prestige classes are a good way to reward PCs and NPCs for being part of a particular organization with unique powers.
Prcs are also a good way to emphasize certain aspects of a class. Take the Shifter, a prc which gains enhanced wildshaping abilities at the cost of spell use. Prcs can be used to make weak concepts viable. Take the int-based fighter, usually far inferior to a fighter based on any physical stat, but with the Duelist prc, it becomes possible.
Prcs can also make certain multiclass combinations that were feasible in the past (wiz/clr) possible again with a prc like Mystic Theurge. They can also emulate "kits" from previous editions like Thief-Acrobat. Finally prcs can be really "out there" and contain bizzare powers that you might not have dreamed of, like the oozemaster.
The well-built, balanced, and flavorful prcs are the reason I like them, and the reason I think it is a good mechanic. I could say the same thing about feats, but noone argues whether feats are overpowered (as a whole).
Technik