> you can arbitrarily render a React root anywhere

I think this exact capability is what led to such strong React adoption. It's almost viral within an application. The main application that I worked on a few years ago was on Backbone / Marionette and we evaluated React by writing several small but complicated nested components, with state controlled by existing mechanisms. After liking both the flux application architecture and the ease of gradual transition, we pulled the trigger on React. It slowly ate our application from the inside out, but never forced our hand.

React isn't "lightweight" (JSX being the primary hurdle), but it is very easy to adopt incrementally!