I queued for the Ahune fight the other night on both my 85s. My druid is currently off-specced into bear, which is my preference. (I spent most of T11 off-specced into resto, which I'm not nearly as happy playing as.) I was happy to jump in the queue as a tank and be rewarded with my instance within a few seconds. So was the one guildie I was grouped with.
This meant we had three puggers.
My guild is a small one consisting primarily of people with relaxed playing schedules, so guild-only 5-mans are uncommon. We mainly log on to raid, and anything else is happenstance. When I do 5-mans, I usually pug, and I'm fine with it. I think pugging makes me a better player due to the unpredictability of what I get.
Anyway. Ahune died. Loots dropped. All was happy in the world.
My guildie and I got on our alts. I queued as healer on my paladin. Guildie was dps. And we waited, and waited, and waited. And after 9 minutes (as a healer!) we finally got in.
Then the tank, who'd clearly never tanked Ahune before, got himself killed by trying to tank the boss (standing in bad stuff) and ignoring the adds which killed everyone else. The tank went off-line rather than face the embarrassment of a explanation on how the fight works.
Which meant we had to wait again.
And it occurred to me, that if I had a third spec... I would have tanked on Gillien.
Back in WotLK my paladin's dual specs were holy and prot. My holy spec pulled double duty for PvE and PvP, but the two specs were close enough that this was "okay" even if it wasn't ideal. Many times, in order to get the faster queue, I'd tank. It got to the point where I was tanking 5-mans more often than I was healing and I'd opt for waiting a couple minutes in the queue just to get a break from day-in-day out tanking on two 80s.
But in Cata, I found the difference between a reasonable arena spec and PvE holy spec were quite a bit different, with 7 of my talent points rearranged to give me access to a variety of talents not present at all in my PvE spec. They're good talents too. They've come in handy multiple times, whether helping my teammate burn someone down with Exorcism spam or getting an extra Word of Glory heal from Eternal Glory.
I use my PvE and PvP holy specs often enough that I don't want to sacrifice one for a prot tanking spec, but I don't mind tanking. I would have gladly tanked Ahune the other night, but I wasn't going to pay 100g in respecing forwards and back for the privilege.
It occured to me that Blizzard is trying to find ways to improve 5-man queues, to the point of offering additional rewards through the Call to Arms, and it makes me wonder if they've considered the potential tank population that just ran out of specs, seeing as all tank classes are hybrids.
I know they want specs to be of a lasting impact, to be meanful. I can understand why they wouldn't want a hunter to have BM PvE, MM PvE, MM PvP, and Survival PvE (which my guildie has said he would do in a heartbeat). To keep this on the topic of too few tanks running 5-mans, why not make the third spec a dedicated tanking spec? If it's not tanking, you can't have it.
I'm sure it would be annoying to implement something just for that consideration, but let's say the third spec can only be used for tanking and people take it for tanking. I can only see it increasing the number of tanks, since people who tank instances already aren't going to stop just because they freed up one of their dual specs, and people who would consider tanking but can't because they ran out of specs, might actually tank.
I don't know how large the "I ran out of specs population" is, but Blizzard, you have one here.