After this week, I'd say that Willow's days in Salem are really limited. I wouldn't be surprised if she were in the next pink slipped group.
After Shawn kicks her out of his apartment, and she goes Pyro on him, she can count herself OUT of Shawn's life for good. I can't see her being worked into any story lines with other characters, so I would have to say she's due to be out really soon. If she were to be pink slipped before Christmas, that would make for a really AWESOME christmas present for Shelle fans.
She could be kept on to find out her back story but if they decide to keep HER while axing Lexi, Stephanie, et al, it would make no sense. No matter HOW CHEAP she may be. That would be the extra few bucks that could be invested in someone who we DO care about.