This looks interesting, but you run the risk of new developers sliding down the learning curve of your codebase. In my experience, rails partials and render calls are always a stumbling point for entry-level devs. The prospect of replacing html tags in a JS app with additional abstraction is a dangerous one indeed.
The most compelling case for Polymer (in my experience) is CSS encapsulation.<p>When explaining to other devs that Polymer makes it possible to create rich tags like the existing video, audio, and progress tags, it's much easier to grasp.