The other exception is that if you can use sp in game it will causes an exponential rise in sp gainage. The more sp you spend in a game allows you to get more sp and so on. In other words if a player is winning his sp will be higher than others allowing his sp to increase faster than others giving a more definite victory from the get go.
A down fall of subtracting the sp as Pendragon hinted at is the sp per game would most likely fall very close to 0 this would happen due to the fact if you don't spend your sp you will lose because your opponent is now stronger and anyone who is about to lose could spend all their sp to spite their opponent.
My suggestion to fix these issues if this idea was to be implemented is to take a middle road. I suggest that half of the spent sp is saved therefore if you and your opponent spend all their sp the winner gets half of their sp and a fourth of the loser's sp; the loser also gets a fourth. All sp not spent is dispersed as it is normally done. The reason for this is it would account for the rise in the amount of sp in the game while saving the match from ending with no sp earned.