-50% potency for all skills for 15s would yield lower dps than just having to rebuild stacks regularly which defeats the purpose of using the stance.
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Why not Fists of Fury!? ;p
Granted, 15 secs would be a bit long. (Cant edit on phone)
Popping a stance to have it sit idle for 10s isn't exactly what I'd consider a useful stance even if used strategically, and the only place it'd really be viable is during Twin. I'd rather build stacks on my own with an attack speed boost, than have a stance automatically put me at 3 stacks with no effort.