It all depends on what you're doing: PvP or PvE, or both and too cheap to respec daily. loljk
Just ask yourself what's my target: killing 1, 2, 3, 5, opponents or a massive front-line in a group?
SL/SL is proficient (thus popular) for PvP: Arena/World, and moderately well in BGs, because of dmg mitigation. I say moderately well for BGs because warlocks do a great deal of DPS but can out perform itself with another spec suited for large group assault/defend. I personally just go SL/SL for everything cuz I'm too cheap, and the reason you don't see many Affliction/Destruction hybrids is because once a melee gets a hold of you then you're toast; nah, more like a peanut butter and jelly sandwich that's been in a brown bag lunch for 6 hours.
Anyway, that was a lot of babble and lacked depth but I hope some of it helped.