As far as I can tell, the primary reason for not having a rating system for modules is fear. Module developers in particular are concerned with the fairness of ratings. They are concerned with "gaming" of ratings. They are concerned that inexperienced or "dumb" end users may unfairly give a bad review of a module simply because they don't understand how to use it. These are all reasonable concerns. But they are concerns shared by other OSS projects as well. Sure you will see "bad" reviews, giving a module the lowest possible rating along with some inane review such as "tis modules sukcs BEWARES" :) But who cares, it's just noise that will be drowned out by valid reviews. It works for other OSS projects, and it can work for Drupal.