I don't think it would ever work, but maybe Skynet really isn't that smart if it has taken so many times to kill the Conners. Sarah is just way too paranoid, and John is just very, very lucky (because he has been very, very stupid in these seasons). Skynet has shown the desire to try different things and methods though, so maybe it figures that it can't hurt. Or perhaps she would serve as one more distraction, futile as the effort might be, which in Skynet's logic patterns would be one more reason that a terminator could succeed.