Actually the main problem of thieves stealthing in WvW is that “player culling” prevents them from being redrawn immediately when they leave stealth, so other players are effectively being attacked by a still invisible thief (their perspective not the thief’s) without being able to see the thief and respond or defend themselves.
Until ANet resolves or mitigates “player culling” this will continue to be an ongoing issue with stealth based thieves.