That was what Metamorphosis was supposed to offer - a 'Oh crap- nothing is tanking the mob anymore - better get something in there fast' button. Maybe if they 'fixed' this to do that job properly (I can just dream of making Meta a permanent 'lasts until cancelled' transformation...), it'll make the Demon tree more popular again cos right now, I've been floating between Demon and Affliction while I'm levelling and I find affliction much better - can take down 4 or 5 mobs easily at once just by dotting them up and rotating the dots. With Demon I find more than 3 mobs and it gets 'tense'.
No-one is going to invite a warlock to a 5 man, or a raid, because if the tank dies, they'll be able to send another one in. No-one really plans 5 mans in particular on having the tank die during a boss fight, and during a raid there's always at least 1 OT to cover this.
These days, the only thing that matters for 'pure' dps classes is what's in Recount. Until Warlock can match, or better, what other classes are reporting in Recount, they'll always generally be one of the last classes to be picked.