I actually run 850k Health on my Wizard, with 16 Million Toughness. Yeah. It takes a lot more for me to die than anyone I party with. I just stand and deliver while being barraged by tons of puny demons~
The thing you must realize, is you can never get 100% reduction, so when you are surrounded by a large amount of enemies, you're taking that 50,000 damage before healing, quite a few times a second, and in higher Torments, you won't be able to keep up a high Healing, High Toughness and High Damage. If you just go for healing and toughness, you lack the damage to "Kill Quickly."
I can bump my toughness to 22 million and only go from 950k Damage to 880k, and my life goes up to 960k, If I need to. Think of this as well, if you can balance resists, armor, and health like I did, then it's a lot better to be slowly whittled away, because eventually, you Pot, boom 60% of ~900k healed back, and they will slowly bring you back down, which by then, Pot, boom 60% again.
Now, I do have a little life on hit and after kill, but Life on hit gets nerfed at 70, so it heals not very much, and on kill... Well if you're fighting a boss like Maltheal, you're going to die quickly by only relying on Armor and AR.