Using Polymer(...) instead of document.registerElement(...) means that for anyone to re-use a component I build, they will need Polymer as a dependency. There will likely be more web component based libraries popping up, so then I'm going to need to download half a dozen identical libraries to use different components for my site? Then there's hacked components like React that don't even use the web components spec...
Yes, to use any component you will need that component's dependencies. That's just true of any software component. The Polymer() call has nothing to do with it, as in the end it just calls document.registerElement(). Raw elements that call into a template library in attachedCallback() will depend on that template library.
