I have done wedding, portrait, architectural and construction photography for some years now. I wouldn't consider a system for work that was primarily aimed at people trading up from compact p&s and trading down from entry-level DSLRs to something lighter, smaller and cheaper with many or most key components of the system based on moulded plastics.
The m4/3rds lens bodies are not built as strong as top 35mm lenses (though some are pretty dang close) because they honestly have absolutely no need to be built as strong. The smaller and lighter the lens internals are the less solid the externals need to be. For instance, my 45 f/1.8 doesn't need to be built like a brick because it is so light that even with a weaker body it would take less damage from 10 five-foot falls onto cement than a big L lens with a much stronger body, like the 35L, will take from just one such fall. And they all get minor scratches and marks equally as easy, but because m4/3rds lenses are usually smaller and lighter they are less likely to seriously harm each other if they are bouncing around in a bag together.
I have used Nikon, Canon and Pentax SLRs and Nikon and Canon DSLRs and currently use Nikon D800 and D3 bodies and Nikon lenses. With all due respect to m4/3, the robust build that you expect from Nikon and Canon simply isn't there. I don't have the time to baby my photo gear, it just gets slung in bags and has to survive some less-than-ideal handling.
Back when I had my 5d2 (I have a 5d3 now) I dropped it 3 feet onto cement with a Sigma 50 f/1.4 attached. The Canon broke and had to go in for repair (the lens was fine). I am completely certain that my E-M5 would handle the same fall with 25 f/1.4 attached with barely a scratch. It is built WAY stronger relative to its mass. The 5d2 would have to weigh probably 15 percent more and be noticeably thicker to have similar relative strength.
There are also the issues of dynamic range, noise at high ISOs and control over depth of field where m4/3 still falls far short of full frame.
Go to this page:
Olympus OM-D E-M5 Review: Digital Photography Review
Now add the 5d3, the D800, and the 7d to the list to the right of the E-M5. Now look at the graph and take your foot out of your mouth.
As to high ISO noise, the E-M5 is better than the 7d for sure. And while the 5d2 has better High ISO
NOISE performance it actually has similar high ISO
PERFORMANCE. That is a subtle, but vitally important distinction. Color accuracy, dynamic range, and tonal range are all just as important as the actual noise at high ISO's, and the E-M5 easily beats the 5d2 in those areas. This offsets the actual noise difference and makes it just as good a performer at high ISO's..... To the point that while I almost never used my 5d2 above ISO 3200 (less than 1% of my photos were above 3200), I have set my upper auto-ISO limit on my E-M5 to 6400 without any concern that I will not get acceptable results.
"control over depth of field"... I keep reading that on these forums and it never makes any sense to me. I have just as much control over DOF with my E-M5 as with my 5d3. Yes there are some things I can't quite achieve with 4/3rds that I can with 35mm, but there are other things I can't do with 35mm that I can with 4/3rds. In the end it all balances out almost exactly and neither has more control in total.
I like my Panasonic G3 and small collection of native m4/3 lenses (Pana and Oly) but I would not entertain using them for paid work. It isn't impossible, but you would have to work within their limitations and take care of them to an extent that is mostly alien to working shooters.
Limitations or advantages? I frequently can't shoot my daughter in low-light with my 5d3 + 50 f/1.4 because the DOF is too narrow for it to find focus. In the same lighting conditions my E-M5 + 25 f/1.4 has a considerably easier time because the extra DOF makes focusing much easier.... And then the shots comes out better as well because more of her face is in focus. This would not be an advantage to some Pros (studio pros who fully control the lighting for instance), but it is a huge advantage to others, like wedding photographers who have to shoot moving subjects in dimly lit venues.
