You're right, a board does move faster when the base is flat on the snow.  Problem is you can lose control that the edges give you.

Let me let you in on a little-known secret of the pros (especially boardercross racers).  There is an easy way to go fast keeping flat base and stay going straight and in control.  Take your back foot and go to an edge, slightly -- very slightly.  Now take your front foot and put it flat so it is not edging at all.  Yes, you will feel the board twist between your feet.  That's how you know you are doing it right.

That will get you going straight and fast and in control all at the same time.  HANG ON!

FR - 8