I explained this better in response to someone else. But more and more lately raids are moving from the 2 tanks for every fight model to the 1 tank and 1 melee dps who off specs as tank for 2-3 fights per tier. What this translates to is 1 tank role, 7 dps and 2 healers per 10m. The numbers scale a bit differently for 25's but as I don't raid 25's I can't tell you exactly how they scale, just that even in 25's there are rarely 2 dedicated tanks -- as most fights are now designed around 1 tank. Which means compared to the healer corps, tanks only occupy 10-15% of the raid composition where as healers are still making up 20-25%. Which is why it would make sense to have more healer combinations than tank combinations.

On a 10m heroic level most guilds shave from 3 healers on normal content to two healers for heroic content. Some even down as low as 1 healer. {Heroic Rag, Heroic Ultrax} While this isn't necessary it can and is done by some guilds. But at the moment 1 or 3 healer comps are rare and most if not all guilds focus on 2 healers.

Using the extreme version of a 10m comp where you have a 1 tank fight -- we see that dps occupy 70% of the raid comp healers 20% and tanks only 10%.

I am not sure exactly how many specs there in game outright -- as whatever number we come up with will likely have a few specs excluded due to the "pvp" nature of those specs {re: Frost Mage, Sub Rogue, etc}

Suffice it to say when all is finally shaken out 5 tanking roles is already excessive. If you bring this to six you are seriously pushing it. Especially considering that this would FURTHER inflate loot tables.

I wasn't trying to crush dreams. I'm not saying that by the time the next xpac after Mists rolls around it wont be an idea whose time is come. I am just simply saying that atm it doesn't make sense for Warlocks to be tanks.