I'll take a stab at it.
I have 85k files or so. I rate songs bc it can be helpful to find what I want to play, esp within given genre[s].
I have a background in radio so I settled on this:
1 star = crap, shit, I hate it.
2 stars = neutral, not worth playing.
3 stars = i kinda like it, want to hear it 25% of the time.
4 stars = I like it, want to hear it 33% of the time.
5 stars = I love it, want to hear it 42% of the time.
Think of the percents as representative of a given hour.
Ratings are very useful for reminding u and letting u find things quickly. In apps like winamp u can sort by ratings and make playlists based on them, and even use math equations to create playlists based on the percents I mentioned above, (a user can set any percent, those are just the ones I use)