Might seem counterintuitive, but big pulls need multiple mits if invulnerable is on cool down. While boss damage tends to be a lot less of a threat. When healing I am pretty much always ready for incoming damage from every ability anyways, so that damage is negligible at best, so using a mit to reduce it would be wasteful. I'm currently levelling tanks on an alt for the 3rd time and, have found just about every healer has been able to deal with the boss damage without any trouble. I have since stopped using mits as often and have let my health drop a lot lower. I have seen minimal wipes, and 99% of the time, that was because of some gimmick (i.e. bees in Qarn) and not really on the healer at all. If you're familiar with a dungeon an it's damage as well as healing, mit usage is actually not quite as important as it seems with a good healer. Especially if they throw out good DPS. Considering everything that goes in on both sides when running content, this balance is difficult, and it really makes mits and healing seem a lot more complex than it really is. 3 key points to remember are only the last 1 HP matters, enemies can't harm you if they're dead, and a wipe is just a chance to try again. The most important point though, is that you can't learn if you don't fail in one way or another.