In our campaigns, casters tend to single-class and meleers tend to multi-class. Personally, I find it hard to stay with a single class for the simple fact that prestige classes usually give more than they take. For instance, in a Greyhawk campaign I play in, I have a Cleric 6/Radiant Servant of Pelor 1. I'm giving up an average of one hit point per level, and in return I get Radiance (all spells with the "light" descriptor double in range), Extra Greater Turning (five time per day), martial weapon proficiency, a third domain, Positive Energy Burst, better healing powers, and on top of all that I still progress as a cleric with regards to saving throws, turn undead, and spell levels. Seem over the top to you? It does to me, but hey, the DM allowed it, so I'm running with it. I lose out on 13 hit points over the lifetime of my character. Big deal. Given the option of taking a prestige class, I can't justify not taking one that makes me a better cleric.