The reason to run Mental Misstep in Team America is clearly spotremoval, since Swords To Plowshares and Path To Exile really can ruin your day. Also, Force Of Will is rather bad against the more aggressive decks packing these cards, so you can now comfortably board out your Force Of Wills without having to be too afraid of your opponents' spotremoval. If these decks start running Mental Misstep as well, that's another thing, but between Stifle, Wasteland, Daze, Hymn To Tourach and all your other stuff, it will be rather hard for them to resolve two of these cards while still maintaining aggression. Apart from that I don't think Mental Misstep is too strong in Zoo anyway, so I'm not afraid of that.

