They can catch on stuff. Ground/obstacle clearance is really important. My wheels are often whizzing past rocks with mm of clearance, or even brushing againsagainst them. And I'm often dropping down stepped terrain, which means the pedals cranks etc get close to the ground, you don't want anything else that could cause contact.
Faff. Offroad cycling is tough on bikes. When cycling with buddies, you don't want to be the one asking everybody to stop whilst you sort out some bike problem. There's enough going wrong with chains coming off or snapping, gears and brakes being knocked out of alignment, punctures, etc. Anything attached to your bike that doens't help with propulsion is a gimmicky waste of pedalling effort, and just counts as something unnecessary that can go wrong. If there's more than three or four of us going cycling, we split up into mini groups, just because if the whole group stops when anything goes wrong we'd never actually get back before dark. Kick stands can go wrong, and don't help with propulsion.. their extra mass actually slows you down!
Pointless You should expect to be falling off your bike from time to time anyway, so it's going to end up on the floor whether or not you had a kick stand attached. So, just put your bike on the ground if you're stopping for a wee in the bushes :)
I cycled up snowdon (the highest mountain in Wales) last weekend. I estimate that for a not over weight kickstand you'd be looking at 1lb or approx .5 kg. With a height gain of about 1000m, that would be about 5000 joules or 1250 calories extra energy I would have burnt. And that's just the total height gain and doesn't include bouncing up and down individual boulders.
I suppose if you're trying to lose weight then that's about the only reason for a kickstand on a mountain bike, but you might as well wear a rucksack with a rock in it so you don't reduce the ground clearance or reliability of your bike :)