Personally, I think keeping him is a good move and even if they had Stollarz still, I would prefer Knight be given the chance to backup Bob. Is Knight a world class goalie? No, but in seeing him play previously, he has more than good potential to be a solid number 2. I do agree that the salary is fairly high for a backup, but it's Zito's money to spend, not mine.
Bobrovsky is also hardly a spring chicken, so while he survived a grueling season, and a playoffs in which he was fairly busy, it may not be the case in subsequent years, especially as he continues to age. But age aside, no player is immune to a freak injury - look at Ben Bishop for TB years ago - getting injured at the worst time. Sure, Vailselevsky was in the wings then, but in my view, better to have a backup that has developed in your system, rather than bring in a veteran backup who may or may not mesh with the team and the defense in front of him.