Personally I think ratings should not be linked to comments. It would make the productratings app simpler, and also allow for some changes to the way the comments system currently works. Making the rating work like "most sites" would improve it a lot as well - what I mean is that a site user should be able to click on an icon or graphic of some sort which indicates their rating choice (and use AJAX to post it so they aren't pulled away from the product page). The site developer can provide the look and feel of the widget, if they want different than a provided default.
Comments can be tied in using django comments by the site developer (separate from the productratings app) or whatever method they want. Heck, people could install Disqus to use as their comments system. No reason for it to be integrated into this app, the way I see it.