SPS staking is required for tournaments to help keep bots out of the tournaments and encourage investing and / or re-investing in the game. If you've looked at the SPS white paper there are several parts in the where future SPS rewards will be based on staked SPS.
Once the DAO gets going voting power will also be based on staked SPS.
So, I think SPS is for the DAO, but how is that related to the game/gameplay? For Tournaments, wouldn't the right path be to limit by card ownership?