I prefer not to multiclass unless there is something specific I want from a dip. If I am playing a roguish character, I will stay rogue until I switch to Swordsage for higher-level maneuvers/stances. If I am playing a warrior, I might take 1 level of barbarian, then go straight Warblade. In general, I prefer to single-class as much as possible.
However, there are some classes that I HATE to stay in, like sorcerer or cleric. I want new abilities every so often, and higher spell levels don't cut it after a while. Classes with bland charts like those tend to have as many easily-acheived PrCs as possible, like sorcerer/fatespinner/fiend-blooded/abjurant champion.
Honestly, no caster has a reason not to go into abjurant champion eventually.