I never understood what people use their hands for so much while snowboarding that they need "mobility". I've worn nothing but mitts for the past 3 or 4 years and never thought to my self "man i wish i could use the dexterity of my finger tips right now". All i need my hands to do is strap in and grab my board.
unless you experience it its hard to say. Everything from reaching into your pocket to grab you pass, ski lock, lens cleaner to making adjustments to your bindings or boats.