Gate is 6 mana, and can help your opponent as much or more.
Yes and no. If they flip a permenant it helps them as much or more, however if they flip an isntant/sorcery it doesnt help them in the least bit, plus when they flip an instant/sorcery you know what they just drew. I never said it was perfect it is a suggestion.
Take into account that they can use their tops as well to put the wanted card on top, not to mention the dear sphinx or vault/key, plus, in the mirror its not the best card. By the way, including it could mean to include sensei's and probably removing null rods and could help your opponent to get out of the lock with something like: "oh lucky me, i revealed a mox and i played it for free and skipping your chalice@0.
My humble oppinion.