In reply to crayefish:
Sometimes people will solo or move together over steep snow slopes but it may not always be quickest. By the time you've coiled your ropes up and put them away only to have to get them out again and re tie in when you get to a tricky bit, it may well be faster just to stay roped and quickly run out rope lengths.
Moving together also requires absolute confidence that your partner won't slip and pull you off. A weaker partner may also be happy to have a top rope even if the other climber is happy to solo.
Also, when you set off up a snow gully you don't actually know you aren't going to be able to get a runner in, often you can.
Finally, snow belays won't necessarily fail. Falls on this terrain may be more like a fast slide. With a well placed deadman, a good stance and some dynamic body belaying there's more chance of stopping a fall than if you don't belay at all.
There are no hard and fast rules, its a matter of judgement.