I can't say that I recall any instances where the game specifically tells you that you ought to only use certain stances for tanking. Calling Shield Oath or Grit "tank stance" begs the question, in a way. The tooltips just tell you that they trade damage for more enmity and mitigation.

Stances are generally about trade-offs, so there's usually an opportunity cost associated with keeping one active. On MNK, do you want to take less damage, move faster, or do more damage? You can't do all of these at once, so you have to pick. Do you want your NIN to Silence or Stun? If you're open minded, you can find uses for each.

On a tank, you should know how much enmity you need and how much mitigation you need. If you do not, you will misjudge the trade-off and either die, lose aggro, or do negligible dps. If this is overly difficult, there are plenty of jobs which don't require this kind of decision making. Unfortunately, at the moment, none of them are tanks.