The notion of enhancement shammies having limited offtanking capability is a relic of the original WoW when paladin/shaman was limited to faction (and in the original game, paladin tanking was just as limited as shaman tanking).
The prospect of enhancement shaman tanking is irrelevant to the game as it stands now, and would require overloading a single tree with two roles, which is a design they're even moving druids away from in MoP.