[zem]

there are some great frameworks that do do one language everywhere (my top choices would be om and hoplon from the clojurescript camp, and opa from the custom language camp). there is something to be said for being able to seamlessly pass data around between the frontend and backend, and letting the language and framework handle the low-level details for you.

[sbov]

I've always thought this would be nice, but whenever I actually design the theoretical code that would use this, the savings is much less than I would have thought. Real world actualities like security and privacy can either remove many of your gains or, if you ignore them, leave you with what I'd consider a buggy website.