It is needed in pve. We do not have an instant spell to cast while moving (can't rely on procs, sw: p hits like a wet noodle), other casters that we compete with can cast while on the move - locks via talent, shamans via glyph, mages just use scorch. Boomkins have similar problems as us.
It's our 10k sw: ps against spells that hit 5-10 times harder.
Still, the main problem is our gear scaling, not dps while moving.
^ PvE pov