The confusion might come from the fact, that destruction is DEFINATELY the best RAIDDPS spec the warlock can have, currently. So, when you reach around 1300+ shadowdamage, you should spec destro for some of those fun 8-10k shadowbolt crits in raids....

But for lvl'ing ? Felguards, hands down. And yes, keep them mana-fed through lifetap, and drainlife yourself when you need and it's just gonna be a freighttrain through a china-shop.