I think they got Bishop as insurance. At the time Lehner had played just 9 career NHL games, now he's up to 10, and he's only 20 years old. If Anderson doesn't come back anytime soon, and Lehner hit a rut, they'd have to rely on Auld, which would basically mean a ruined season, despite the team looking pretty good this year. With Bishop, they can now rely on whoever is hotter of Lehner or Bishop. Lehner has looked great, but relying on a 20 year old rookie to start night in night out is potentially asking for trouble.