There's nothing wrong with simplicity but if you add homogenization to the mix then that's where problems start. It can get to the point where defensive cd's should just be put on Role Actions list to save on skill bloat since all of them are mostly the same and on the same cd timers, similar to Rampart. Here's an example of simplicity and homogenization using tank stances in the current game:

Simplicity:
PLD/GNB = +20% defense, 10% healing, enmity increased
WAR/DRK = +20% HP increase, 10% healing, enmity increased

Homogenization:
PLD/GNB/WAR/DRK = trait, stance only gives enmity

Tanks having 2 different stances is simple to understand but is enough to differentiate for casuals and can gives options to barrier healers. Homogenization is what we have now where you could technically put one tank stance in the Role Actions and it would still function the same, but that would be too obvious. The tank stances aren't stances but enmity buffs and nothing more.