Where have you been hiding? There's been a virtual explosion in the number of languages that compile down to JS[1] in just the past 1-2 years, and TypeScript is probably the youngest of them all. CoffeeScript[2] is another big one, I believe it's much more Ruby-like. There's even 7 projects involving Haskell.
First, running the same UI code on the front-end and back-end will enable the best performing apps across platforms. Imagine being able to run a rich client on a powerful PC that just talks to an API and does all the rendering as a single page app, while simultaneously being able to serve up pure HTML/CSS to mobile devices and search engines. Being able to run code on the server or client as needed solves too many problems to ignore.
But on the other hand, javascript flexibility and lack of type system will inevitably cause problems as the codebase grows unless you have a good lead developer to define standards and an iron-fisted approach to code reviews.
Personally I think coding standards are a stretch with a language as wild as javascript, and a higher level language to enforce certain things is the best way to reconcile these two problems. I can even imagine supplementing with something like (for instance) a Haskell library that lets you generate JS code for interfacing with the back-end in a provably correct fashion, and plugging that into the UI code.