Sacrifice is obviously the biggest benefit against another player, especially a hunter. Demonology, imo, is the only spec this would completely ***** over. Personally, I have never dueled with anything but my imp, generally because I've always done my lock duels while waiting for an instance run.
Battlegrounds are a completely different story. A warlock immediately has a bullseye over his/her head, especially in Alterac Valley, when the locks will have felguards, unstable affliction, howl of terror, death coil and any number of other nasty spells they might have (and seed of corruption at the 70 battlegrounds). That said, the hunters in battlegrounds are generally BM or MM spec, and sometimes will team up to use Silencing Shot and Bestial Wrath in conjunction with each other in order to ensure that the lock is dead before it has a chance to do much of anything. (Myself, I consider my job done if I can use instant Howl in the middle of a group and send them scattering while the tanks destroy what's left over the next 8 seconds.)
As per duels, there's generally an easy way around the pet thing anyway, depending on your level. Death coil immediately will give you enough time to cast fear (2 sec for death coil, 1.5 sec cast Fear), then dot dot dot (and dot again if you can get out UA and/or Immolate as well). Howl of Terror if need be, then Fear again and drain. Easily slaughtered rogues and druids this way, ime (unless, of course, undead anything, but that's a different story).