They are, unless you want to avoid duplicating logic, like I said. That's where most of the benefit of server-side rendering via JS comes from: you can render the initial page on the server, but then have the client-side framework take over on load, all from the same framework and with very minimal code duplication. Otherwise you have to write most of your rendering logic twice, in two different languages/frameworks.
Re: benchmarks, I don't know what that has to do with anything. JavaScript (via V8) is much faster than Ruby and Python, and yet Rails and Django are still massively popular. For most apps, using a super fast backend language is not a concern.
Not if you need both server-side rendering AND a rich frontend. Unless you want to duplicate logic between two frameworks.