First, I assume you're talking about the "new" mirrorless designs like the micro 4/3s Olympus Pens and Panasonic G's, Sony NEX, Samsung NX, etc and not the DSLR 4/3s cameras (Olympus E5)

IMHO, the micro 4/3s cameras represent the best combination of size and capability. Far better low light image quality than any compact, and much smaller than any DSLR/lens combination. Kind of the "jack of all trades, master of none" solution.

From a technical perspective, there is really no need for a mirror and all the moving parts (and associated size and cost) in a digital system. Those things are only necessary when you have a "single use sensor" (aka film), not a reusable one like a digital chip.

So, I guess the answer to your question is if you wanted to only carry one system that was smaller than your DSLR and more capable than your compact. For me, it came down to DOF. The small sensor, and associated infinite depth of field, of compacts was too limiting, and a DSLR (and more so the lenses) was too big.