As soon as you start having changable lenses then ports become an issue (cost, availibility, packability)
I have gone the Olympus route (E-PL3 in an Olympus housing) and can use the 60mm macro and the 9-18 behind the Zen port. or the 14-42 behind the standard port with a wet lens for macro.
I only have live view and that does not seem to cause any focusing issues.
I would like the 8mm fisheye but thats another port and the hand luggage is getting very full already.
On the surface I find the pen series Ok but not as nice as my D300.
I don't shoot video so cannot comment on that.
The 8mm panasonic (Lumix) fisheye is very small and light and so too the Nauticam 3.5" port for it.No weight or bulk problems for travel.
When I first made the step from a compact to the EM-5 I regretted not having the ability to shoot everything on the one dive, macro and wide using assorted wet lenses.I then reminded myself that the reason for the change was to get better image quality with good glass and more camera options. So I got the 60mm macro and the 8mm fisheye. I don't have a good wide zoom yet so the 12-50 in the macro port is a, not particularly satisfactory, substitute.
If having to make a decision on what type of subject you are going for prior to the dive is a major problem then I suggest sticking with a compact rig.