It would help for a few reasons:
1. It would reduce the knowledge barrier required for new players to get into PvP (e.g. they could actually see which spell was putting a specific debuff on them).
2. In certain zergfests, it can become almost impossble to tell who is casting what.
3. It would reduce the advantage Asuras have over other races in PvP.
4. Honestly I feel like, at times, the animations in this game are not clear enough to justify not having enemy cast bars (certain spells do not give enough warning).
I actually see no real reason for there not to be enemy cast bars.