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.

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.