Google's ability to indexing content is only part of the story. Google cant link to your content if its not deep link friendly and will rank it poorly if you content loads slowly. Both of which SPA are often bad at delivering.

I demoed how big the performance difference can be at Manning's recent powered by JavaScript conference https://speakerdeck.com/markuskobler/react-the-importance-of...

No, since one of the bigger benefits of isomorphism is having fast initial page loads.

The standard SPA experience of having to wait for JS to download and execute before the page appears is terrible for large classes of apps (ex. media), and isomorphic JavaScript solves that while preserving the benefits of SPAs.

There's more to isomorphism than SEO; one code base that allows for both the initial html to be delivered without needing to run js and responsiveness of a client-side app is my favorite feature of isomorphic javascript apps.