If two players are doing nothing but spamming Fireballs then there is indeed a problem, but it has nothing to do with SP pots. It has to do with using inefficient spells.
While I understand that you feel there is an issue that needs to be resolved, I can't honestly say that I see it in game.
The number of players proportionally who play in such a way is rather tiny. The vast majority of players simply don't down pot after pot simply so that they can spam more fireballs (To use your example). They instead use pots sparingly if at all and only occasionally will they drink a few if lag spikes happen or mobs get lucky shots in and party wipes are close.
Changing SP pots when there isn't actually a need for it will simply irritate players and prevent those occasional party saves which will further irritate people. To compensate people might choose to simply drink pots as a "safety net" of sorts and not let themselves become low. This would actually lead to more pots being drank.
Or they might simply say "You know what? Sod this, there's other games I could be playing which are more fun."