Well, rainbow stalls of all kind (arena or non-arena) can use Mindgates to a great effect.
Anyway, it is true there is not any reason to keep its playing cost so high and it should indeed be reduced by 2.However, since its ability can be REALLY powerful (gaining two and three times your opponent's deck) I think its cost should definitely NOT be reduced. At worst, you pay 2
to see your opponent's next card and filling your hand with a card (slighly countering Ghostmare and Sobra strategies) which is not bad at all if you have any quanta to spare.