With ReactJS you get to code as if everything is a component, why use polymer now instead of that?
@jastanton phrased it nicely in another comment here: https://news.ycombinator.com/item?id=7971847
It's much safer to do it this way than rely on polymer. IMHO. When web components really are here for everyone, you are still stuck with polymer libs and tools and all the code is bound to them. Who knows, polymer might be considered bloat then?
That said, these aren't really the same kind of libraries at all, so I'm not sure why this is such a x vs y thread. Ideally, react and web components are fully composable, and it doesn't really matter where the element came from.