There is nothing sexy about it. The lens is disproportionally large and heavy. If you find that looks 'sexy', then putting a 70-200 2.8 on it will look sexier.
Better yet, a big white.
Once it comes out I'll have to try the EOS-M2 on my 400f5.6 (I guess that's just a "medium" white in this case).
Using a compact camera on a large lens still makes sense, the lens is only "large" in one direction, it's like a tube. It's actually not that hard to fit just a tube into various small spaces. The 400f5.6 with a T3 mounted actually fits in the bottle holder on my backpack very well. It's not overly large or unbalanced (and would only be better with a smaller camera).
Putting my 5D2 with battery grip onto that setup not only doubles the weight, but it's virtually impossible to find something to carry it in.
It's the same thing with the 5D2 and battery grip on a Pancake, that combo takes up virtually no more space than the body itself, so there's a ton of places it will fit. Put an 85f1.4 on there and you're back to carrying around something that takes up the same three dimensional space as a toaster.