Since the discussion have rolled a bit offtopic it acually interests me now ;P
I'd have to say though, the "lack" of tanks is not due to a lack of choice or even that people dont want that "Leading role"
If you look at 5mans vs raids its simply a numbers game.
1 x 5man holds 1 tank.
1 x 10man holds 2 tanks.
1 x 25man holds 2 tanks.
The above holds true as a general rule of thumb. Which is why there can feel like a shortage of tanks alot of the time in dungeons. There's simply not a demand for tanks in raids high enough to translate over to enough tanks for 5mans when people queue for such things..
Thats my take on it anyways