It's also not any different than Facebook Likes or +1's. Ever read about bogus Amazon reviews? They're practically a meme unto themselves. There will always be people gaming any system.
Add the ratings/reviews if you think it will help your users make decisions. That's what I did. On sites where it doesn't make sense, I don't add them.
(Also, it's a lot easier to get legitimate users to rate things than to write reviews, as I have found. People don't mind clicking on stars, but it's not always easy to get them to actually WRITE something, unless they're ticked off)