Well, there is such a thing as pulling too much. And that is entirely on tank if they do it. There is also such a thing as pulling too little (slower than you have to be, not as much challenge = not as much fun, etc). So it is the tank's responsibility to get the pace right.

Secondly, there are things others - including healer - can do to make mass pulls go well:
- Be careful when you cast regen/medica2. Don't put these up when or just before tank is pulling. This would lead to you getting agro quickly, before tank gains good control.
- If you do get agro, get close to the tank.
- Everyone get close together (healer and DPS). This is both for making it easy on tank to regain lost agro, and also for Cure3/medica/succor/whatever.
- For WHM, use Holy. For example, Swiftcast > Holy > (normal) Holy causes the 2nd Holy to land exactly when the stun from 1st Holy is wearing off. Total of about 6 secs everything stunned. The damage is just gravy. Of course, this requires all the enemies to be close together. And you have to make sure you can switch safely between healing and Holy. It is kind of a skill-intensive tactic -- some beginners stay away from it.

Finally, the whole "tank holding agro" thing can be done easily and reliably just by the tank in most situations. Even with just the bare minimum of assistance from healer/DPS (ie, ignoring most of the above I typed). I go in there on i99 WAR after having done it far more times than I can count... Big trash pulls are just never even close to a problem. However, if you get a lower geared and less experienced tank that wants to romp through (this is how they learn, hopefully), then there are likely to be some mistakes made.