It took me a long time to start using cleats for XC riding but now I would never go back to flat pedals. I have egg beaters and they are really easy for clipping in and out, and are practically indestructible in all weather conditions. The pedal/cleat combo's appear to get clogged up with mud pretty easy (I saw plenty of people at the Otway Odyssey trying to unclog their pedals with a stick).
If you wish to have the use of both cleats and flat pedals, I would recommend getting a pedal spanner and swapping between them whenever you need. Hope that helps.
Yeah, I did a fairly muddy ride recently with 6 guys. 4 of us had shimano SPDs, and 2 had crank bros. 1 was the bare egg-beaters, the other had the ones with the flat cage around it. All of the Shimano riders (including myself) get really clogged up. I had 3 gumby stacks cos I couldn't even get my feet out of the damn predals. The two crank bros riders had no problems at all.
That said, a friend of mine has gone through 3 sets of crank bros in the time that I have had 1 set of bottom of the line shimano SPDs with the plastic cage. And he also warned me that the cleats wear out real fast (whereas SPDs last forever).
The MTB shoes usually have chunky tread or even screw in studs, that make walking up trails easy (even carrying your bike - damn Otway Odyssey!).
Clipless (of any type) improve power and control a HUGE amount. I have had near stacks when the ability to lift the bike while pedalling has saved me from rather unpleasant introductions to tree roots, boulders, and so on. Not to mention being able to go flat out over rocky, bumpy trails without having to waste muscle power holding my feet on the bike.
So, get some, set them so you can click in and out easily, ride with them a LOT, practice clicking in and out, deal with the occasional stupid stack at the start, and you will grow to love them.