Why would you want to shoot a Canon full-frame camera that costs more than twice as much (3.700 € incl. the lens for the Canon, 1.330 € for the OM-D), weighs a kilo more (1.609g vs. 587g), is much bigger and bulkier and doesn't even look as good?
Simple, because it gives technically better images. How much better lies in the eye of the beholder and whether it outweighs the above downsides as well.
Full-frame gives you better noise performance at higher sensitivities, about a stop in this case. With 4 times the sensor area (2,0 crop factor) I would expect the Canon to do better, the OM-D really has pretty little noise, but the Canon is still superior. I don't give too much on noise. Out of my over 100.000 images I've shot not a single one was ruined because of noise, so what.
The reason I shoot full-frame is you get a big finder. The 5D finder is smaller than of classic film SLRs but much bigger compared to sub-frames. The OM-D actually has a pretty nice and big electronic finder, the 5D's is just bigger.
The second reason, probably the most important, images look like they did with film in terms of depth of field. With full-frame and a fast lens you can nicely blur backgrounds and emphasize your subject, it's much more difficult with sub-frames. To show you what I mean I shot both cameras wide open, the Canon with a 35L at f/1.4 and the OM-D with the 17 1.8 at f/1.8, its widest setting.
Olympus
Canon
You know what I mean?
If that doesn't impress you, forget full-frame and use the cheaper sub-frames, the OM-D is a wonderful little camera, I really like it. If images count, like on a wedding, I grab the Canon. Its images are just superb.
Happy shooting ...
KIKI