if you're talking about the 'newschool' tuck of sitting down on your tails (although I'm pretty sure skiers have been doing it for years), the reason I do it now and then is just because it's fun to do.
As far as I know, the way a 'racer' tucks is probably the most affective way to tuck, as far as having control, and minimal wind resistance.
Just do what's fun though, that's why you're on the hill.