with such a powerful effect, i think it should have a negative effect on the users also.
because the capturers of the flag will be able to use potions and stuff;
- a constant health degen should be applied to them but not at a very fast rate
- or creeps do double damage
- or armor deficiency etc.
since the opposing team will be somewhat forced to not use potions, the capturers will be forced to heal every now and them at the expense of gold at the same time.
This negative effect will also be something both teams would think about before capturing. But force them to think fast before the other team take the upper hand.