Throughout the history of photography there have been different formats. Where a new format offers something not available before it has survived and even been successful. Leica introduced 35mm and its compact size and acceptable image quality carved out a place amongst the technical cameras and medium format. APS (film) didn't really offer any advantage over 35mm and died.

Regretably I feel the same is true with 4/3 and full frame. The 4/3s can be a little smaller than say the 5D, D700 or Sony FF (all of which have a detachable vertical grip) although the E3 isn't that small but the difference in image quality, for many, would make the difference not worthwhile.

Of course, for the present, it is APSc that is Olympus's competitor and there the difference is much less. However my personal opinion is that we will end up with compacts and full frames and possibly medium frames if Hasselblad do bring out a real 645 but that's not of interest to many.