Not sure if you’re talking about the same favourites module?... Solspace Favourites lets members create a list of permanent favourite entries for handy reference - like a shopping cart. I can’t see how a non-member can have this type of functionality.
It also has features like “People who bookmarked this, bookmarked this and this etc..” Each ‘favourite’ can be associated with members and via the admin you can browse members favourites…
Ratings doesn’t seem to have member data associated with it, and as far as i can see is more open for abuse - hence making it member only at this stage. I don’t want to have guests entering their name and email to make a rating also, they may as well signup and become members in my opinion…
Again, I may be wrong as I’ve only skimmed through the docs and done a quick implementation at this stage.