Let's have a poll on this topic.
Question: Do you not run as tank in LFD/LFR because?
A. Other players keep trolling me (eg. pet taunts, Priest Lifegripping me for fun etc).
B. They too easy for me, I'm too powerful and awesome!
C. They're boring and I don't need anything from it, not even valor points.
D. I prefer running scenarios.
E. I find tanking intimidating and prefer to DPS or Heal instead.
You forgot F.
F would be "I run with my guildmates instead."
The only reason I'll ever tank an LFD run would be for the satchel. If I need valour, I'll tank for my guildmates, because I'm guaranteed a 5-12 minute run (never had a run go over 15 minutes and that's cause I d/c'd for 4 minutes).
As for LFR, I only go in there as offspec, because I would rather get my valour from 5-mans with my guildmates and friends than do LFR for valour. I don't need any tanking gear from there, that's for sure. I'd tank it if I could get offspec gear while tanking, but that's not currently possible.
Also, our guild is a 25-man guild (with at least double that in active people). We're more than capable of running a guild LFR should we choose to do so (and in fact, we did at the start of the tier).