Psion-- DMs flex their muscles because they can. It's allowed. the DM is the guy who understands game balance, (well most do anyway) and it is he who decides whether or not a prestige class will work in his game. Me, i don't allow them, thinking that characters are better off staying with a class and not adding levels to a character in different classes. i have a lot more respect for a player when i see this on his character sheet: fighter 17, than when i see ftr 3/sor 2/hexblade 4/rogue 4/assassin 2/kensai 2. You create a mish mash of various classes that don't have any sense being together, and the character becomes unable to do even anything useful. I;ll give you an example of something in my game. one of my players started a fighter, went up in that for a few levels, then switched to sorcerer. he took four leels in sorcerer, and then asked if he could prestige into kensai. I looked it over and said, sure, no problem. He used a halberd as his main weapon. He gained a few speceial abilities, thought he was cool, but his character was 2nd rate at best. He took the spinning halberd feat, something he had been wanting since he saw it, and it was useless against the monsters i was throwing at them. his final PC looked like this: FTR 3/sor3/Kensai4. He was so below par in everything, he was useless to the party, and he died when he tried to storm the courtyard of a hobgoblin stronghold. He was butchered by 4 raging hobgoblin barbarians. He would have lasted a lot longer if he was just a 10th level fighter. After that, i decided to just take prestige classes out of my game, and if someone wants to be called a kensai, or flame mage, or templar of the order of Bahamut, i just go ahead and let them use the title but they remain fighter, wizard, or cleric.
The classes in the core book are balanced, fair, and are perfect for the game. Imagination makes for prestige classes with what is there, noto some underbalanced set of new abilities, feats, and other stupid nonsense. If you don't have enough imagination to make your character cool in one class, go back to video games and leave D&D to people who can actually think and have imagination.
flame away, 'cuz i know it's coming.