Otherwise your React site will take the form of a black hole as shown on slide 6 of my Isomorphic Rendering with React talk.
Google and Bing both crawl Ajax pages. But when they crawl Ajax pages, they have to spend more time doing it, meaning your site will not be crawled as quickly. And that’s pretty much where it ends. There are thousands of other crawlers out there which will not be able to access your site any longer, and that’s not necessarily a good thing for your site or the Web.
Once you start building React apps without SEO friendliness it’s almost impossible to go back without some extensive overhauling or creative workarounds.
If you’d like to understand your situation and options to address SEO issues with your React site or app please contact me to schedule time to talk.