Lookups for syntax and the like is very annoying. However, learning new languages is all about the principles you pick up.

I'm a better iOS developer today, because I learned how awesome blocks can be from Ruby. Same goes for meta-programming.

Languages usually have features that span everything you do in that language. Getting comfortable with blocks in a language that uses them as a primary/native data type shows me how useful they can be in a language that uses them more cumbersomely.

That's how I've benefitted from learning multiple languages. As well as the fact that I will avoid Java/.NET because they and I don't agree! :D