If the provider is in a position to provide a prediction, then the rating system is useful. For example, on Netflix I used the Hated It, Didn't Like It, Liked It, Really Liked It and Loved It system. When they predicted a star rating, it was pretty close. When they said we predict you'll give this a three star rating(which is probably well below the "average") that was generally a movie I liked.
But my point is that for me, it didn't. Netflix's system was good enough to take into account that people have different systems. Thus when Netflix says "we predict you'll give this 3 stars", that means it was a movie I would like. That might mean you gave it 4 stars or 2 stars or whatever, even though you liked it as much as me. They made my system the only one that matters, as long as I was consistent. Reviews in aggregate are pretty much meaningless, but a good system weighs that problem in.
