I think it's a neat idea, but ultimately, it would probably be unneeded. If implemented well, the way I'd see it is tanks would have 2 different rotations: tanking and offtanking (which would just be dps). The tanking rotation would incorporate the enmity spenders such that you would never actually lose enmity. And offtanking (dps) rotation would simply max out dps, enmity be damned. The idea of "are you good enough to ride the enmity line" would be solved almost instantly by the community with an optimized rotation, I would guess. It would, I suppose, raise the minimum skill required to tank effectively, though.

I could see this idea being really good, though, if they ever moved away from the Trinity model, turning all classes into "role and dps", where role is tank, healer, or ??? (buffer/debuffer maybe?).