I own two "full function" multi tools (Topeak Hexus 16 and a Park Tool I-beam mini w/chain tool) and a small 6-hex key fold out (Topeak mini multi-6) that is about the size of a pink rubber eraser. The bigger tools are nice to have, but I can't honestly say that I've needed all the extra tools whilst on the side of the road. Between the mini fold out, a patch kit and a set of levers, I have everything I need for quick repairs. For anything more than that I have my cell phone and a full set of tools at home.
That being said the one real advantage of having the larger tools with you is torque. When something is slipping, you just can't get any good leverage on the small tool to do something like cinch down a loose shifter or seatpost clamp. Maybe enough to get you home, but not enough to fix the problem.