The thing is, all things are rarely equal.
First off you have lens resolution, I've found most of my lenses for my 16MP OMDs kept up with or outresolved my (very good) FF lenses on my Sony A900, which was 24MP. So cropping heavily with a FF camera requires some extremely good optics to keep pace.
Also, when you crop you increase noise as well (when you compare at a fixed output size). Obviously the per-pixel noise level doesn't change, but the amount of noise you see for a given print size does.
So you lose the noise advantage, lose the DOF advantage (if you're cropping heavily you're further back meaning wider DOF), you have to have extremely good lenses to hold up in terms of resolution, you need to have a current generation FF camera (5D2, D700, A900, etc are not much better than current get M43 sensors pre-crop and are worse post-crop), so you're looking at a significantly higher cost, as well as larger size and weight. Heavily cropping any camera with a separate PDAF focusing sensor is going to magnify minor focusing errors which plague pretty much all DSLRs as well (not important for certain types of shooting like landscapes but very important for portraits, wildlife, or anything with narrow DOF).
Certainly a current gen high res FF camera like the D800 gives more options when cropping (when you have a lens sharp enough and have technique sound enough to actually get the detail), but you lose out in so many other areas that I can't really see the argument.
Web resolution and very small prints are probably the only realistic cases where you'll be able to do a 2x crop from a FF camera and match the output of the current 16MP m43rds sensors.