I remember Asonimie posting something like this.

He wanted the warlocks to run as either Demo or Destro initially, until they reached a spell power point. Not sure what the number would be in today's arena or battlegrounds, or what that number will be in Cataclsym. I believe he wanted 2900SP when he posted.

The idea being, with that amount of spell power, in 1-2 globals, the target will die from instant cast dots alone. So the tactic is to apply the dots and then fear the healer, or something similar. With enough spell power, the target is dead in the duration of the fear. Without enough spell power, you're not going to beat a decent healer who is focusing on keeping your target alive.

In battlegrounds, most of the time healers won't be nearly as focused as they are in arena, and it will be much easier to take targets out. So depends on the medium of play and your goals with the team. In the current game, arena is worthwhile as that is the best rewards -- even if you just lose your 10 games (trying to win, to improve your skill) each week; in Cataclsym, with the best rewards from BG's or Arena, you might be better served by just doing rated BG's.