"it does not help for fast prototyping or small apps imo"
I can understand this sentiment---React really is meant for serious apps. There is a decent learning curve. There is small overhead. And though it now only requires just a tiny bit of setup thanks to create-react-app, it might easily require a very significant change to workflow habits. (And the fact that CSS is still held by some as an ideal way of maintaining UIs rather than a temporary requirement given what browsers can consume in 2016 is a testament to how how hard habits can be to change. But I digress!) If you're not already knee-deep in it, React is almost certainly overkill for a marketing page or a mostly static form-y interface for some rails backend.
But just to give an alternative opinion: I think React is great for prototyping. The big reason is that React allows you to easily build and use custom abstractions. When you get down to it, that's sorta the whole point of the library.
If you need to do something---maps, a text editor, a flipping clock---there's a decent chance a catalog like js.coach will already have a React component for it. And at just a single line import away, unlike old jQuery "imports" which typically required more setup which then was more fragile once you wanted to make a change somewhere else (like, you know, renaming an id).
And over the years, like many others I'm sure, I've evolved my own toolkit which happens to be perfect for prototyping. I can vertically align and just by passing "center". I can add a modal simply by typing right next to the button which opens it. I can add data persistence and real-time updating mostly just by typing and tweaking some config for the new app. I could go on and on. And these are just little personal things which make life a little easier. There's also projects like react-flip-move which smoothly animates transitions in a line of code, react-sortable-hoc which gives, in a line of code, a sortable with pretty good touch support and essentially infinite lists......
Obviously in principle you can get these benefits in other frameworks. But React is one place where lots and lots of people are actually contributing to the ecosystem.