I've been using the Miche system for a few years on one bike along with more typical thread-on cogs on other bikes. Heres my take on it.

1. If you do lots of skidding or forceful backpedaling, I wouldn't recommend the Miche system because their is a chance that the minor amount of back-and-forth play will loosen the lockring. I'm not saying that it will happen, but it might. I have noticed slight loosening of the lockring over time...but very very minor amounts; however I don't skid or forcefully backpedal much with my Miche equiped bike.
2. If you ride a lot of miles, or put your DT through gritty nasty conditions often, the Miche system is more cost effective because the cogs are cheaper and can withstand more wear. By that I mean that when one side of the teeth are worn, you can flip the cog around and use the other side.
3. The Miche system is obviously most beneficial if you change gear ratios often...like at the track.
4. If you are basically a normal person who doesn't ride excessive numbers of miles, doesn't change gear ratios often, and uses a front brake, the Miche system has no real benefits IMO.