Sigh. I swore since this thread popped up i wasn't gonna read it and for damn sure wasn't going to post on it......yeah right. I was fooling myself. These posts are going to kill me one of these days. They do help me refine my arguments, though. 
the Prestige class is an inarticulate rpg tool. In fact it causes more grief than it adds to the gaming experience. I'll even go so far as to the use the official term...El Sucko!!
It doesn't even come close to the accuracy that feats, multiclassing, and even templates provide for creating a unique character that individualizes your characters. Because prestige classes are static, they are invasive as all get out, and they do a crappy job of individualizing a charracter.
They do sell books, however, so thats why the persist.
Some are just put together badly, they don't even bother me so much. Its the ones like Guild Thief and Harper (anything) that irratates me and, in my view, ippotomizes (sp?) what is so ridiculus about prestige classes. The Harpers are not a drilled group of troops who are all the same. In fact, i don't think any of the harpers i've read about (in sourcebooks or novels) even fit any of the prestige classes they have. Hell, the Harpers themselves can't agree on who or what a Harper is. So why do the prestige classes exist? Because they sell books.
Guild Thief. please. Although its pretty much the same argument as above, the Guild Thief is so damned generalized that its barely different than the Rogue in the first place! You can already do it with the class(es) available!Which means it just doesn't need to exist. Wanna join the thieves guild? Go to them (if you can find them), pay your dues, and join up. They are not going to ignore you because you have 5 ranks of move silent instead of 6. Especially if you have other useful abilities that the requirements of a prestige class can't ever allow for. So why does it exist? Because it makes money, not because its needed. Imagine a Thieves Guild campaign where everyone works for the thieves guild and so everyone gets levels in the prestige class. Now not only is anyone now NOT unique or individualized, they actually begin to look quite a alike. I just don't see Thieves Guilds like that. At all. What happens if you are already in the Thieves guild when the prestige class comes out? Are you suddenly kicked out of the Guild, or do you get handed the "Well you don't NEED the prestige class to be part of the guild, a lot of the guild members don't have it!" bunk. So why the heck do they exist then??!! To sell books.
Then there are the prestige classes that are wrapped around a game mechanic that isn't presented in the core classes. Such as the Assassin's Death Attack. Of course if you take out that one class feature and the class implodes. Add it as a Rogue Special Ability and the Assassin Prestige class becomes a rogue with the poison resistance feat. Crap prestige class. Cool Rogue ability.
Or there is the Spellsword prestige class that if you make its channel ability into a feat, you don't need the prestige class......Oh wait, they already did that....its called Arcane Strike.
The Weapons Master.....so getting weapon focus, weapon specialization, improved critical, and all of the other feats for mastering a weapon doesn't already make me a weapons master? Sez you, bub!!
Prestige Classes exist because:
1) They make money
2) Players/customers have been conviced that if they don't have a Guild Thieves prestige class they aren't a Guild Thief. Even if there is no such a specific animal.
3) They make money

the Prestige class is an inarticulate rpg tool. In fact it causes more grief than it adds to the gaming experience. I'll even go so far as to the use the official term...El Sucko!!

It doesn't even come close to the accuracy that feats, multiclassing, and even templates provide for creating a unique character that individualizes your characters. Because prestige classes are static, they are invasive as all get out, and they do a crappy job of individualizing a charracter.
They do sell books, however, so thats why the persist.
Some are just put together badly, they don't even bother me so much. Its the ones like Guild Thief and Harper (anything) that irratates me and, in my view, ippotomizes (sp?) what is so ridiculus about prestige classes. The Harpers are not a drilled group of troops who are all the same. In fact, i don't think any of the harpers i've read about (in sourcebooks or novels) even fit any of the prestige classes they have. Hell, the Harpers themselves can't agree on who or what a Harper is. So why do the prestige classes exist? Because they sell books.
Guild Thief. please. Although its pretty much the same argument as above, the Guild Thief is so damned generalized that its barely different than the Rogue in the first place! You can already do it with the class(es) available!Which means it just doesn't need to exist. Wanna join the thieves guild? Go to them (if you can find them), pay your dues, and join up. They are not going to ignore you because you have 5 ranks of move silent instead of 6. Especially if you have other useful abilities that the requirements of a prestige class can't ever allow for. So why does it exist? Because it makes money, not because its needed. Imagine a Thieves Guild campaign where everyone works for the thieves guild and so everyone gets levels in the prestige class. Now not only is anyone now NOT unique or individualized, they actually begin to look quite a alike. I just don't see Thieves Guilds like that. At all. What happens if you are already in the Thieves guild when the prestige class comes out? Are you suddenly kicked out of the Guild, or do you get handed the "Well you don't NEED the prestige class to be part of the guild, a lot of the guild members don't have it!" bunk. So why the heck do they exist then??!! To sell books.
Then there are the prestige classes that are wrapped around a game mechanic that isn't presented in the core classes. Such as the Assassin's Death Attack. Of course if you take out that one class feature and the class implodes. Add it as a Rogue Special Ability and the Assassin Prestige class becomes a rogue with the poison resistance feat. Crap prestige class. Cool Rogue ability.
Or there is the Spellsword prestige class that if you make its channel ability into a feat, you don't need the prestige class......Oh wait, they already did that....its called Arcane Strike.
The Weapons Master.....so getting weapon focus, weapon specialization, improved critical, and all of the other feats for mastering a weapon doesn't already make me a weapons master? Sez you, bub!!
Prestige Classes exist because:
1) They make money
2) Players/customers have been conviced that if they don't have a Guild Thieves prestige class they aren't a Guild Thief. Even if there is no such a specific animal.
3) They make money