Casting any sort of heals mid-pull when the tank isn't on death's door is not what people are supposed to be doing. Tanks have more than enough defensive cooldowns to be able to handle themselves until they gather up all the enemies, as well as a Panic Button in case things get dangerous. Casting regen in the middle of the tank pulling helps literally no one, since it's not an instant heal and all it does is give the healer aggro.

If a healer gives you regen before you've hit all the enemies, they're a bad healer. Full stop.