Well since it is a class system and not a point buy I frequently have to multi to fit my character concept. In greyhawk I had a rogue/wizard I was going for something like a arcane indiana jones for the character, and I have a rogue/cleric. This was becuase it was an "evil" campaign and my god was pyremius god of murder, poison and fire bwahahahahaha. Anyhoo the clerics were a bunch of sneaky coniving bastards and rogue levels were almost a prereq if you read the description, eventually the plan was to take a level of assassin to finalize my concept. I was a punk but dam playing up the fire aspect was a blast especially since we had a fire oriented sorcerer on the team.
The indiana jones one rocked, yeah my spellcasting took a slight hit, but man with spells enahcing my skills I out rogued the rogue, except in sneak attacks.
About the only characters I haven't multied are the psions I played. Maybe I should of since they were a little weak, but I had a blast with the shaper.
Oh yeah I didn't multi my cleric of st. cuthbert he just kicked far to much but single classed for me to wuss him out with a fighter level or something.