Personally, I don't understand how you DO die as a sniper. Ambush crit=insta dead mob (for anything less than a silver) and a non-ambush crit is still generally enough to follow up with an auto attack for the kill. Then two snipes will finish most everything else off. Sitting on a 31 sniper, full marksman so far. Literally never died due to mobs, unless you count when running through tunnels, pulling everything.

You're doing it wrong. Just went ahead and proved this on a level 34 Mara. You only get the heals on a crit, which I did. I even took video if you need to see it.