Correct me if i'm wrong, but i believe that once you reached the 7.000 rating points barrier, you rating might decrease if you lose a match, a high level museum surely will help you to lose just a few points.
However, in the way i look at it, museum is only useful to huge leagues (such as thai league), where you start at such a low level that you can't bring not even 12.000 people without some decent rating, top level teams might find it useless.