OK, I agree with this approach (entering the personal ID), however if it is hidden somewhere in the Preferences, it wouldn't be very convenient. Imagine we are three people at home using MB, every time the next one should change this settings.
Also, it must be visible which ratings are currently displayed, maybe the stars can be preceded by this ID, I don't know.
On the other hand, MB is currently not really a multiuser program, I mean it cannot be started multiple times by different users. So, it would be better if I install it three times, so everybody can run it's own copy, with its own settings, then the suggested approach can work well.
If some day Steven decide to rewrite MB and implement multiuser support, then everything will come to it's place.