i think it is a good system but i think that you cannot really go and rely on it ..people can just ask some friends to get their trust level higher ..or am i wrong? 
You aren't supposed to rely on it, it's a tool, nothing more.
So what is the point of having one? If trust system doesn't work, then it just misleads people into trusting the wrong individuals thinking they are safe.
Because it's better than nothing. In the end, only you are responsible for your decisions and the risk you take.