I guess PvP might not benefit as much from haste as PvE (haste is pretty much the only arcane stat in PvE) but I'm starting to switch out some of my crit gear for haste for arcane PvP. Missiles benefit from haste and you have to have huge haste to even get an arcane blast off in arena and crit is getting near useless to me since I run an arcane / shatter spec for PvP (80%+ base crit chance on frozen targets).

Really it's up to playstyle but arcane / crit is slowly getting me no where in arena now adays.