Now and then I decide that I really should reforge my weapon, then realize that I've not done that yet because Bryn'troll has nothing to reforge. Stam, strength, and... lifestealing? That's not a stat, it's a proc. Those clearly aren't stats. Or are they? Once again, I'll make possibly useless comparisons to Torchlight. Which you should try.
WoW has more than a few lifestealing weapons, enchants, and spells. Warlocks have drain life and deathcoil. I won't count them since that would be as absurd as claiming polymorph as a potential gear stat. Alright, quick list, may not be complete: Lifestealing enchant, Skullforge Reaver from Stratholme, Demonfork from Lower Blackrock Spire, Syphon of the Nathrezim, and a list of all weapons with "steal" in the name, which does have some mis-hits, stealth, and the effective duplication of Shadowstrike.
So the stat is there and has been giving some sort of value. That means we could see it more often if we wanted it.
More than a few things go against this. It goes against Blizzard's attempt to simplify stats. Though the DPS gain per class/spec should be somewhat straightforward, it's still an added bit of complexity. To make it worse, it's a melee-focused stat, but melee have the agility-strength divide, so the weapon would either have to be gimped for one side or have nothing but lifestealing. Then within the agility community there's the melee-range split, and guess what, this wouldn't be very useful for hunters, unless on a ranged weapon, in which case it's useless for melee.
The usefulness of the healing aspect would depend heavily on the situation. It is likely to save downtime while soloing and could be a slight help against stray damage in PvE, but it's not going to save anyone from what would otherwise kill them, or at least so rarely as to be insignificant.
But maybe it could be a gem. A meta or perhaps one-per-set prismatic. Torchlight uses it as a gem and sometimes on weapons, which are named as Draining, much like WoW has Owl or Bear. My experience has mostly been as a Vanquisher where my constant kiting meant that I rarely took damage, so a bit of healing here and there would make up for it. On the downside, my constant kiting also meant that I wasn't able to autoshot very often, so it wasn't triggered much. For a Destroyer or a melee-oriented Vanquisher it might have been more useful. But I liked the idea of the stat and found it to be fun. After all, there's a strange thrill to not being a healing class but seeing that health bar go up anyway.
Blizzard's attitude towards lifestealing would need to change for it to be a real stat. Currently it's a gimmick, barely noticeable except that for some reason a huge lifestealing proc was put on an axe, Bryn'troll, off the first boss of 25-man Icecrown Citadel, making it the easiest 264 weapon, easier in my opinion than most of even the 251 weapons. I doubt this will happen, but what better time for senseless, useless speculation and theorycraft than around an election?
Would you want to see lifestealing given a more common place in WoW?