I play Horde, and almost all horde I come across have warmode ON.
One exception: warlocks, almost always warmode off.
I guess that says all you need to know about warlock PvP.
He man I'm hardcore, never have it off, but you are right in rarely ever see warlocks with it on. I know that if I face another lock, mage or priest I have a pretty good chance of a good fight. Anyone else and I get blown up in a global during a stun.