You seem to have a good understanding of how it works. I wouldn't necessarily call it useless since it can help you compare where you might have a good matchup against a certain opponent, ie. your SG usually gets a 10.0 rating and his an 8.0.
I suppose there's also salary and DMI that can help predict that, but those are based on the player's best position and most teams have at least one guy playing "out of position", say a listed PG playing at SF. On the other hand if rating were based more closely on game performance, I would call that pretty useless because you can already see all the other stats the player got and make up a rating number based on that however you like
