Let's talk numbers. Truth is in numbers. First, let's discuss the differences.

On my warrior+druid team, I can deal with mana burn. For the most part, mana burn doesn't really get cast because my warrior is busy giving the priest an enema with his sword, but if it DOES get cast, I'm usually breaking LoS so often that I'll only get hit with it once a match, and usually it doesn't matter because the priest will be dead for mana burning instead of healing himself.

Also, I can deal with mana drain. A warlock will not be shadow-bolting (or incinerating) while channeling a spell, and since every sl/sl lock has the improved distance talent, I use this channeling as a very expensive root, of sorts. For instance, last night we faught a high rated priest+sl/sl team, and we won. The priest was on a pillar enjoying his enema, and my warrior was just pummeling his heals, mortal striking, like you do. Anyway, I would sit behind one of the posts on either end of the bridge, toss out my heals, and when the lock finally got line of sight, he was standing on the ramp cement, so I would start running (just not quite under the bridge, but offset a bit so I can easily and quickly get up the ramp on the other side). Basically, the lock would continue channeling until I was half way between the pillar and the ramp on the other side, which was usually ~3 ticks (600 mana). I'd throw out my hots on my warrior, dispel the curse on him, etc, and wait for the lock to show up to rinse-and-repeat. I didn't have time to drink, obviously, but my mana consumption was still a far-cry better than the damage that the priest was eating. Long-story-short, we won that match pretty steeply.

We don't always beat priest+sl/sl, but it's easier than every other healer+sl/sl (other than shamang, which is probably comparable to priest). ENEEWACE... viper sting... man...

Okay, I have pointed this out: mana burn makes you stop dps'ing, you aren't mind flaying, mind blasting, etc. Mana drain means you aren't throwing out sbolts or incinerates or fears. Viper sting, on the other hand, lets the hunter continue dealing damage, trapping, etc. I don't care if it DOES have a cooldown, it's still the best mana drain in the game.

After 2.4 patch with 400 resilience:

Mana drain: 240 / tick; 5 ticks (but today's ptr posted it had been nerfed from 300 to 200 / tick, so maybe it'll be 160 / tick)

Mana burn: 800 / burn (costs 355 mana for the priest)

Viper sting: 371 / tick; 4 ticks (plus the hunter will still be dumping damage into people, so you have to burn mana healing while stung).

If the current ptr sticks, and the warlocks are nerfed down to 200 / tick before resilience, I won't really have to worry about the mana drain any longer, so I can heal to my heart's content. Priests, again, I don't worry about much anyway. Hunters... will still be unbeatable teamed with a healer with cc (like a druid).