Found this great post by GnatB in the Starfall thread that pretty much sums ups my question:
"Incidentally, speaking of BA, is it just me or does the Drain Life skill suck?
(pun sort of intended)
From what I could tell in GA, you do half the damage you normally do, but *that unit* gets it back as health. It's only worthwhile if the unit in question has actually taken damage, otherwise all it's doing is reducing the damage you're doing for no good reason. So it's only worthwhile for tanks. Comparing it to IW, it would seem to fail miserably. IW doesn't reduce the damage you do. IW negates an entire enemy attack, (not just 1/2.. effectively) IW can proc for each and every enemy attack, not just your own."
Guess I'll be feeding the Ronja I leveled.