I have to agree with Zaltyre's assessment. The hag's damage potential as an Act 2 open group monster is severely underwhelming and they are only three monsters! Though the idea of hero lock down is scary, consider the fact that the most recovery they can prevent is four health if all three monsters are on the board.
In theory they can be devastating, but if they are locking down heroes in Act 2, then the OL would be winning with a different open group anyways. Chances are with Lifethirst, heroes will need to perform actual "Revive" actions instead of using skills like "Prayer of Healing" so they can at least get the characters to revive.
Heroes like Rendiel, Aurem (with a healing potion), Andira, Ispher and Serena can easily work around Lifethirst. Also, classes like the Berzerker, and Beastmaster make it less effective.
Cursain