That's likely because 20/41/0 and 0/41/20 are garbage for raiding and instancing, and only moderately powerful in a pvp senario.
0/21/40, 41/0/20 and 30/21/10 will all beat the crap out of felguard builds in raids. Normal mode instancing is about as easy as it gets, a 0/0/0 warlock can do fine in them. Heroics.. you're usually on CC duty. Damage means nothing in a heroic compared to coordination of the group.
And in pvp? 5/45/11 is the felguard spec. 24/37/0 with the same gear will rape a felguard warlock every time. (banish, anyone..?)
There really isn't a kingpin PvE/PvP spec. Sorry to say, but there isn't.