I think you're spot on yeah.

Content aggregators (where Digg, Reddit, and to a certain degree StumbleUpon were the obvious choices) generally use the upvote/downvote tooling to promote content that users think are worthy to be shared or should be seen more; same with comments if they add constructively to the discussion regardless of viewpoint.

The problem with using upvote and downvote style tools to express like and dislike means that you just end up with an echo chamber of "goodthink" and a community that circlejerks itself, where dissenting or controversial but possibly accurate comments and posts never see the light of day.