I have two taunts on fairly short cooldowns. I can throw a shield that generates enough aggro that whatever is not attacking me, will. I have an AoE with a pretty decent range. Add to that my usual attacks of crusader strike/hammer of the righteous and judgement, and aggro should not be an issue, regardless of who pulls. At level 70, very few enemies hit hard enough that someone will die from pulling.
In terms of safely clearing an instance, it doesn't matter much who pulls.
So why do I care?
I could twist together all manner of arguments for why people shouldn't pull if they aren't the tank (or acting at the request of the tank).
Is it a bad habit? Maybe. But if we're learning our habits from level 70 random heroics, we're already in trouble. "It's a bad habit" is a form of the classic slippery slope argument, that once someone does something, it is permanent.
Sometimes it makes me uncomfortable when others pull because they are going faster or going when a CD is down. Is this enough reason? Maybe, maybe not. Is my slow pulling making them uncomfortable? Do they have a buff wearing off? Maybe they're in a rush and I'm ruining their run.
I've always been 'right', not because my instance running was at the ideal speed or because there was some divine mandate that made a tank-led pull always inherently better than any other. But I've always had something on my side, a mystical force which made my opinions worth more than others, regardless of their actual merit. I've been a tank and as long as tanks have been in short supply, I've magically been correct. Supply and demand said that my opinions and feelings were more important than those of other players.
The only ethos of "Might Makes Right" has been replaced with "Market Makes Right", in which certain people who by luck are in greater demand relative to supply have become something more than mere humans, something more than individuals offering individual opinions for individual regard.
In the process, the particular whims of these sorts of people have been carried on and enshrined in the collective concept of good behavior, regardless of the actual merit of the behavior, regardless of whether "good behavior" is actually helpful or whether "bad behavior" is actually harmful.
In the past I've espoused a "let them die" strategy. Do two wrongs make a right? If someone has aggro, I get it back. I'm a tank. That's my job. Why should I suddenly break this rule just because someone broke a rule that doesn't matter anyway? I've said that a tank should pull because the tank knows his (or her) limits. Why not give the same trust to a DPS and if they want a mob, they can have it? It's not "let them die", but "let them live", tank as I would and heal as you would and let's get back to the serious matter of killing actual enemies.
"Their actions make me slightly uncomfortable" is not a good justification to go stage a mini-strike and jeopardize the shared goals of the group.
I still think tanks are the natural leaders of the groups, but maybe that doesn't mean they have to be so damn self-important.