my exp from lvling as reto druid not long ago - got healbot set left mouse button to rejuv -> tried no to fall asleep 15-90 while clicking only mouse left button and occasionally right button to aoe heal -_-
That's precisely what I did in LBRS - me (bear) and 2 friends (ret and tree), all lvl 53...must've took 2-3 hours to clear the place but it was worth every minute. Really brought back the old vanilla feel of epic dungeons full of danger, requiring everyone to be on their toes, and the great feeling after completion knowing that we'd done something really hard.
Even challenge modes don't compare to that, probably because they're just short zergfests in pisspoor dungeons. (the baron 45 and the old ZA bear run were better examples)
As I'm sure has been already pointed out, at no point will healing matter. You can literally take everything except rejuvenation from your bars and successfully heal you way to 90.
The only point at which I can imagine things getting tight is the cata 5mans, but I'm sure that they are nerfed heavily by now to fall in line with the rest of the game's 5mans.
I don't know why you think that. Doing BC heroics at 70 there are plenty of ways to wipe even with decent gear. It's unclear to me how gear shortens fear or keeps you from running into other packs when you are feared.
But aside from that there are plenty of ways to wipe in lower level instances too. And if you have a chain pulling tank you will need good heals, especially below level 60.