Looking through this system, the stances would play a bigger role then the Auto-attack per say.

Examples: End game NM hunting. DD role. you would say turn auto-attack on to allow higher DPS, then stay in a more Aggressive stance. maximizing your DPS.

Tanks would be jumping between stances to deliver WS's and resume the tank role (depending the style of the player) for me Tanking. I would turn Auto-attack off to allow me to time my attacks for avoiding damaging blows (frontal attacks) this may lower my DPS, the trade off would allow less MP for healing, lowering enmity generation of healers. also increase few other aspects on top. though this all depends on the play style of the individual

For solo play, would bring more to the plate. can set auto-attack on. then bounce between stances to max DPS, letting the player have a more relaxed playing field. Same with turning Auto-attack off, giving the player a more hands on feel.

As you stated in last post, there will always be a "set" play style depending on what you are doing. allowing the players to decide what they prefer on how to play. would make the battle system easier to take on, making it friendlier for both sides.