The DOF argument is a valid point (for some shooters)
It's not that you can't get shallow DOF on micro 4/3, you can with very fast lenses.
You never get that focal length compression effect that you do on full frame, it is very different it's not just about aperture or equivalence of DOF but focal length effects.
Put a 100mm lens on micro 4/3 and you have a field of view of a 200mm lens, it's not even close to full frame in that respect (even if you have the speed to achieve shallow DOF) It's true APS-C is a compromise too, just not as much of one..and with 1.5 crop the effects of focal length is reduced but not nearly as much as micro 4/3 is.
It might not matter to everyone though, horses for courses as they say.
For me I find the ability to use my FF lenses on my APS-C bodies (most are useful, some less so) I can run straight into full frame at any time as most of my lenses are full frame. That is a key advantage. If you're in micro 4/3 land, that's it..APS-C users can intermix with FF lenses as and when required. As well as pick up a huge variety of s/h lenses with autofocus, and aperture control.
As for size yes I see for some it matters a lot and nobody doubts that advantage, nor the "good enough" aspect to IQ as well (has to be noted Olympus are overstating their ISO figures significantly according to DxO)
But then again with an APS-C body and a 17-50mm f2.8 runs quite a bit smaller and lighter than a FF body and a 24-70mm f2.8 so it's down to how smaller or lighter your needs are.
Honestly there is nothing wrong with micro 4/3 it's an excellent choice for many. I personally find the prices on the bodies to their biggest problem.
I was playing with a Lumix GM1 the other day, nice little camera the sort of thing you might pick up even if you are a DSLR user. A camera to take out for the day with good IQ.
Then you look at the price tag, and they want higher than APS-C DSLR's for it £600..so you wander off and lose interest. And no need to mention there is no VF either which again puzzled me on the cost of the camera (no VF= cheaper to make)
What's wrong with micro 4/3 is they have to be better priced, smaller sensor = pay less. FF = bigger sensor pay more. Makes sense to me. Small is fine, just don't expect me to pay a hefty premium for it. Ditto on the Sony RX100 II, nice enough camera just seriously overpriced.
Every time I see one of these offerings I just buy another DSLR for peanuts