Sometimes talking about a backlog gives someone direction on what game to play next. I know if I wander into a backlog discussion and a bunch of people start talking about their good experiences with a game I haven't played, I'm more likely to play that next if it's on my shelf.
I believe the scoring system here is just a stepping off point for the actual review content.
Take Homefront for example. Three stars tells me, per the Site FAQ, that the game could be a toss up between enjoyable and not enjoyable depending on my preferences. But, like they say, the stars are based reviewer experience so not reading the whole review is pointless. If I'm stoked about the single player in Homefront and three stars is enough for me to buy the game, I'll most likely be disappointed because that portion of the game isn't very good. Had I read the whole review, I would have known that the multiplayer is stronger than the campaign.
But if it's just one reviewer, how do I know if my gaming preferences will line up with theirs? Well, luckily this site has a great user base who like to write reviews! Homefront currently has seven user reviews in addition to Alex's review. So after you read thorough all those you should have a great idea of what that particular game's strengths and weaknesses are and can decide whether you want to purchase it or not.
GB's scoring system is fine the way it is. Focusing on only that is like finding a gold coin and being satisfied when you could have walked another five steps to find a treasure chest.