Have nice day.
Warlocks are more about controlling the pace of the whole game, be it in BGs or Arena. Your skill cap would be very high and how you survive and dish out damage would allow other classes to finish enemies. Your burst is lower, but your control is higher.
So ask yourself, do you want to dispatch enemies quickly and have twitch reactions, or do you want to control the pace of the battle and multiple enemies at once?
A bad player can do well with a Rogue, a good player is unstoppable
A bad player will fail with a Warlock, a good player will fail with a warlock, a amazing player will do well