The problem in this whole "skill" wonderland is I can't think of a single scenario where the harder tank was the better tank for very long in the many mmo's I've played.
Anyone else?
DPS, it's easier, because you can give them procs. Tanks don't like procs because the class comes to depend on them and suddenly, fights end up hinging on procs. Procs for tanks need to be harmless if they don't happen like "Maim has a 20% chance to make your next overpower free.". If you raid an entire night and that never happens, no harm no foul, but if it was something like "Butcher's block has a 20% chance to slow the target", that starts to become something you want to see more, developers end up framing things with that in mind, etc.
WHM has procs, but whm seems to play in a model where they cast the cure they need to, mp be damned, and cast Cure IIs when Freecure happens to proc. WHM's procs can change the course of the fight if they're having mp troubles, but it's less likely, especially with ballad available.