For me, the world is a big place, and there's room for a lot of stuff out there. If a player is interested in a particular PrC, I'll do everything I can to find a way to fit it into the game - and being (IMHO) an inventive and creative guy, I do pretty well. My tabletop campaign features the very first Spellsword - a warrior mage who is developing his abilities based on a very incomplete understanding of the elven bladesinger art. He'll be quite surprised when he meets a real bladesinger...and so will the bladesingers!
If I happen to know of a PrC that would fit the character, I suggest it to them and if they're interested, find a way to work them into the story. That's what's happening right now in my game - an NPC is basically tutoring one of the PCs in the ways of a certain PrC that the character doesn't even know exists.