The problem comes when people get overeager and don't let the pull finish. Remember that, in the middle of a big pull, we're not gonna stand in place waiting for another GCD, it's AoE on the run. So, if you start laying out heals on me before I have them all grouped up and can ACTUALLY establish aggro, you're gonna have a bad time. Trust in me to survive until I can get them all grouped up. My whole toolkit is about survival.
This should pretty much end the discussion.

I mass pull and don't ask because... why? The vast majority of parties can handle mass pulling with ease.

Honestly, the real question should be, "Why don't tanks ask if it's OK for them to do small pulls?" Because that's the outlier situation that people either want small pulls or can't handle the big ones.