it's been an update or two since I played my warlocks.
all I remember about destructo was being out of mana and life drain didn't heal me nearly as much as it does an afliction lock.
that was a few releases ago ( back when 70 was the max )

I wasn't impressed with my 4xwarlock-priest team that I used RAF to level up ( level 80 was the max) . They were fun until I got to the outlands.
They couldn't take down the outside elite in the pool of aggonar in hellfire at level 60, one group heal from the priest and she had aggro.
compare that with my 5x pally team that did all of the hellfire dungeons at 60-61 ( leveled up inside of the instances ) and that elite was nothing to them.

I find warlocks can top the DPS charts but they have to work at it and use massive AOE such as seed of corruption or any long boss fight with lots of DOTs.
one warlock with 3 shaman, the shaman spells cast faster than anything the warlock has except instant cast DOTs.
for outside trash, the 3 shaman will kill before the warlock can cast his first spell or his first DOT ticks for more than once.

I liked my 4x hunter / shaman team. They did better in dungeons, maybe I just wasn't playing my warlocks well or maybe I should have tried demo spec on the warlocks. 4x and 5x felguards just look impressive.