Conversely it's a bad idea to spread yourself thin in multiple languages but master of none.
Having said that, I'm pretty much tied to Java since 2000 and haven't seen anything wrong... yet. In fact, I'm determined to get better in Java everyday (memory model, JVM, concurrency, better design, etc).
Side note: I'm also learning pure "modern" JS just because there's no choice on the client-side.
Actually, it's quite easy to write for the client in most languages targetting javascript. Certainly it's the case for CoffeeScript, and TypeScript looks even better in that regard since it's a strict superset.
I've done GWT for about 2.5 years from version 1.x to 2.x. I'm not sure I prefer one or the other.
GWT development is a bit painful to setup and to work day-to-day.
It's nice to have the similar Java structure and to be able to write unit-tests to test the app via MVP patterns but I'm still not sure it's a big win for me going forward. Especially when the founding members have left the team.
Having said that, I'm pretty much tied to Java since 2000 and haven't seen anything wrong... yet. In fact, I'm determined to get better in Java everyday (memory model, JVM, concurrency, better design, etc).
Side note: I'm also learning pure "modern" JS just because there's no choice on the client-side.