Oh, hey, as I said, there's no problems with the DM saying "No." I actually specifically said this. The DM's campaign is his and elements within the game should fit. Thus, when the Forsaken elf player in my Scarred Lands campaign wanted a PrC that gave his barbarian divine rage, I said no. Forsaken Elves are forsaken because they don't have access to a diety. It didn't fit with established elements in the game.
But, again, this is world's apart from the DM saying, "You must". When the DM decides that his version of my character is better than my own, that's a problem.
To give a personal example again. In my World's Largest Dungeon game, a player who had gone through four characters in six sessions came up with a new concept. The ultimate saving machine.
A monk/paladin/Pious Templar (eventually) with saving throws absolutely through the roof. Can't fight worth spit, but, man, he never fails a saving throw.
Now, I was rather blunt and said that this character isn't all that good. Sure, it makes saving throws, but, it can't do any damage, which is usually considered a bad thing for a fighting character. Poor BAB compared to a single classed paly, bad monk abilities, it's just a watered down concept. But, the player stuck to his guns and ran with it. And, considering that it wasn't actually violating anything established in the campaign, I let it come in.
Sir Alex the Lucky has become a pretty solid character. Took a bit of creativity from the player, but, he's actually not too shabby at holding his own. Definitely not a bad second stringer. And, he makes the ultimate trap finder.
He's happy, he found a niche and a character that's actually managed to survive a surprisingly long time.
It's his character. So long as he's happy, why should I care? I don't change my adventures to suit the players. They are what they are. It's up to them to figure out how to get their square pegs through my round holes. (Oh man did that come out wrong.)
Now, I'll admit that my game's a fair bit looser than many. I don't have a particular campaign setting really, so, flavour issues are less of a concern. But, I've really learned in the last year or so that my opinions of a given character could quite possibly be wrong and the person running the PC probably knows it better than I do.