I think that DM's should have final say as to what PCs are available, and how many you can take. There are just some PCs that do not fit well with most campains. What do you tell a player that wants to pick up Fist of Hexor, Ravager, True Necromancer or Acolyte of the Skin? These classes don't really mesh well with the standard party set up, and would only serve to destroy any good or neutral aligned party or campain.
You also have to think of a prestige class as the D+D equivilant of a master's or doctoral degree. It is rare enough to earn one, and very few people earn more than one. It is not really a bad idea for a DM to impose heavy Role Playing requirements for earning a PC, or even adding requirements in addition what the books suggest. I mean as players we spend might spend 4-8 (sometimes 12) hours playing in their game in a week, but the DM spends all week creating it for us, and if they think that our Fighter/Rogue/Ninja/Cavalier/Drunken Master is a little to unlikely and has clashing ideals we should listen to what they have to say.
You also have to think of a prestige class as the D+D equivilant of a master's or doctoral degree. It is rare enough to earn one, and very few people earn more than one. It is not really a bad idea for a DM to impose heavy Role Playing requirements for earning a PC, or even adding requirements in addition what the books suggest. I mean as players we spend might spend 4-8 (sometimes 12) hours playing in their game in a week, but the DM spends all week creating it for us, and if they think that our Fighter/Rogue/Ninja/Cavalier/Drunken Master is a little to unlikely and has clashing ideals we should listen to what they have to say.