For someone who saw the DVD's extras, the reason Lucas gives for this Light saber spining is that these two heroes have been forged though the same wars, the same circumstances and their technique is similar as twins look like each other. So the pointless spining is the outcome of someone kinda-facing himself.
This resembles of someone trying to play chess against himself: If you try to play a mind game playing for both sides, the game will always be silly and stupid to the other spectators, and the one who wins is the one you intentially choose to give advantage to.
I must say that even if I found this scene stupid at first, this explanation convinced me, and I came to like the whole duel now that I watch it from this angle.