Actually it does have a dedicated star tracker, and (the system)[https://www.nasa.gov/content/goddard/hubble-space-telescope-pointing-control-system https://www

Actually it does have a dedicated star tracker, and (the system)[https://www.nasa.gov/content/goddard/hubble-space-telescope-pointing-control-system https://www.nasa.gov/content/goddard/hubble-space-telescope-...] is designed to combine gyro measurements with star tracker measurements. Both the Lunar Reconnaissance Orbiter and Mars Reconnaissance Orbiter are dealing with or expecting gyro issues and are currently coming up with and testing control paradigms that only use the star trackers. I think part of the problem with the star trackers is that if you "lose your place" then it's very difficult to get back on track and this could lead to serious problems.