I know, I never can fully explain what it is I don't like about factotums, it's probably just lingering distaste from watching one played high-OP in a low-OP small party...you can imagine what that looked like, I'm sure.

As for casters, they don't alter their roles from magic user to rogue to combat character and back on a round-by-round basis...there are casters that blend those elements, like beguilers or hexblades, but they all have specific setups and strategies for them.