He seems to be a pretty happy iOS developer. He likes Swift as a language (and I must say having used it in frontend dev it does have some very nice things many other languages often used for server-side development don't come with), and he loves XCode as an IDE. If for their personal use-case it does the job and isn't missing major libraries or so, I don't think why such an endeavour shouldn't be supported?

I don't think it's so much about sharing libraries - consuming and creating data is quite different, and backend and frontend work probably shouldn't be duplicated.

NB: Some of those 'features' I mean are: - Strongly typed (compared to JS, Python, Ruby..) - Compiles to binary which could bring benefits depending on your needs (compared to JS, Java, Scala..) - Being very new, there's little 'legacy sillyness' in Swift 3. Comparing some PHP sites to Swift projects (Frontend, we don't use it in any backend applications yet) I can tell you which one I'd rather work with.