And it's easy to tell when a dps is doing bad: Look at the meters. It's not as easy to tell if a tank is doing bad unless you know what tanks have to do on a fight (which usually random people in LFR don't). I was vote kicked from Empress one time because I got MC'ed since the other tank didn't taunt off of me. They didn't realize that it was the other tank's fault and not mine.
Want more tanks? Blizz should make it where you don't have to rely on the other tank to get your job done. Let 2 tanks queue in, sure, but have the 2nd tank there only as a back-up tank (if the MT dies) or picking up adds or whatever.
I'll only tank LFR if I have another tank queueing with me who won't let me get to 5 million stacks of a debuff that makes me take a million times more damage or let me get MCed.