When you find the first source there isn't problem, source exchange makes his magic. But sometimes this first source is incredible difficult to find.
If there is any objection because asking sources that not have the file anymore and so, it can be limited to save a max of 5-20 sources per file, this would be enough, the only problem is to connect to the first source.
Right now isn't implemented because the harm to the network or only because programmers had not enough time to do it?