One reason I didn't was Feign Death.
However, once I did train the spell, it was like a drug with no side effects. One that made you invincible.
Soloing group quests? Have a go and if it doesn't work out, Feign Death and start over. Aggro too many mobs? Feign Death and run away. Dazed off a mount? Feign Death, re-mount and continue. Spot a herb? Let my pet get aggro, gather the herb, trap the mob, Feign Death, mount up and get the hell out.
Unfortunately, my alts, which aren't hunters, don't have Feign Death.... and boy, do they pay for it.
I think, though, I'm starting to break free of my hunter habits and play classes as they should be played. For example, I'm finally confident enough on my mage to pull a group of melee mobs and AoE them down rather than killing them one at a time. My warlock has less downtime than my hunter while farming. Even my druid rarely dies after learning hotkeys for Swiftmend and Nature's Swiftness.
Of course, some things I still can't handle - I tried several times to solo Princess Tempestria on my druid, but always ran out of mana and health by the time I got her adds down and had to give up (I'm still certain it can be done; I'm just not as capable with the class yet... and my unusual spec probably didn't help either) - but my alts don't die nearly as much as when I started.
So, I'm happy I've weaned myself off the addiction. Feign Death isn't a drug any more, it's a car.... and if I'm on a character that can't drive, at least I've trained myself to take the bus!