The benefits - while not of great impact - would teach tanks a that they have to dps at least a little, and dps could "measure" themselves vs others more accurate. You CAN hold aggro with 1-1-1... only while at least one the same iLvl as the dps. At least in my opinion, that should not be possible; it just doesn't feel like pressing 1-1-1 while stance is active should be the bare minimum... that's just lazy. It's similar to a healer spaming cure 1 during a lvl 90 raid... while their target will probably never die, this doesn't feel like the bare minimum either, and more like they are watching Netflix while running a bot program. Well anyway, that's just my opinion.
Another point I heard is that apparently we were hitting enmity cap last expansion. This could lead to e.g. tank losing aggro during Delibrum Savage vs a dps with minotaurus buff - although rare, it did happen. Considering how this game progresses and tanks dishing out more enmity than ever since ShB, it could be a measurement against this soon™ upcoming issue. Well, it's still just a wild guess.
On the enmity generated by using stuff... honestly, I am not entirely sure whether it was actually just 1 point, 1 potency or anything like that. But since its impact is minimal, it's neglectable. Just like the enmity fix on Living Shadow /s
Oh, and should actions do additional enmity as a bonus, it could teach tanks to use their whole toolkit. However, that would mean tanks are getting forced to dps hard to hold aggro vs good dps/strong heals, which could lead to newcomers become pressured. So... eh.
PS: I just checked some enmity values from before ShB. Tank stance back then gave a 2.7 multiplier while reducing dmg dealt by 15/20%, thus an effective enmity multiplier of 2.295/2.16. Even while NOT using aggro combos like Rage of Halone/Butchers Block/Power Slash, tank stance alone was enough to hold aggro in most scenarios! In conclusion, a times 3-multiplier (or even a times 2-one) would've been enough as stances don't reduce dmg dealt anymore... Maybe slap some bonus enmity on aoe skills (reason in my previous posts).
I believe Sqex just wanted to rightout remove the enmity aspect of tanking with the exception of Shirk and Provoke... which is mind-boggling. Aggro management has never been this least interactive.
I repeat myself, but since the enmity bar proves no purpose, Sqex could actually remove Shirk and Provoke, too, and just let the target face the tank with stance everytime they attack. Essentially no difference.