If you're not quite familiar with evaluating players through their skill set, the ratings can give you some indication, but you really need to take into acount what factors influence rating before you can attach value to the number. For example, stamina and minutes played are a factor for ratings, so you need to account for that. If you see a player that gets 15+ rating but only plays 10 minutes each game and has low stamina, you can safely say his rating will drop a lot when he plays starter's minutes.
Also, gameshape is a big factor. If you see a player that plays 48 minutes in each game, you know his gameshape has been bad for those games and if you manage it properly he will get a higher rating than in those games.
There are other factors as well that influence it, so I think that accurately evaluating ratings is harder than just evaluating a player by looking at his skills.