To be as useful as an alternate in really poor weather conditions, the alternate should have an approach with minimums equal or lower than the primary choice. They often do not, especially for general aviation aircraft destined to remote locations. You can't reasonably expect to go missed from the primary ILS to find another one a short distance away. A possible caveat to this nowadays would be GPS-WAAS approaches which are getting pretty common. They are as good as ILS and small airports often have them.