It helps to know more than language, you can more easily talk to more people. It is true with foreign languages, it is also true with programming languages.
First of all it's a good training for your brain. You learn new ways of doing the stuff you usually do. Sometimes you'll find that your common tasks are more easily done in a different programming language.
Second of all when your main programming language gets worn off, you will be able to switch to another one easily.
You will also be able to get along with 3rd party software developers. Knowing limitations and strengths of various different languages you will be able to predict what will be difficult to implement in a given language.
Also remember that programming skill is independent of a programming language. It's like knowing how to speak. If you know how to speak in one language, you just have to learn new vocabulary and new grammar to properly speak in another language. No need to learn how to use your tongue, vocal chords, teeth, etc.