There are some minor benefits to a fix. The first is that in the ice and slush your derailleurs can get stuck, but no real problem there, just don't shift, and it'll be like a single speed anyway. There's the aforementioned issue of weight, but really, in a winter beater MTB, the ounces you'll save aren't going to mean a lot. The only real benefit is this - in extreme weather conditions, your freewheel pawls can freeze up so your cranks just spin, and THAT's a bummer, specially since it's the last type of weather where you want to push, or sit waiting for a lift. I know that this happens, because people report it from time to time on BF, but how often it really happens, I wouldn't venture to guess.

The big reason that most people ride fixed is cos they either like how it feels, and/or want the extra workout. There is a viewpoint that says that fixed is the best way to brake the rear wheel in the slippery conditions, because you really feel what's going on back there, but having no experience, I'm not qualified to comment on that.