In the first decade of my career I wrote software in C++, Java, C#.Net, Ruby, Python, Objective-C, VB 6 and VB.NET. Additionally I developed for ASP, ASP.Net, and PHP based Web Sites as well as scripted in Powershell and Bash. Learning multiple programming languages was very helpful for me during my career as a software engineer.
Here are some reasons that you should learn multiple programming languages.
Trains you to separate the engineering from the language
Learning multiple languages teaches you to explore design patterns and engineering best practices that aren't platform or language specific. Being a great software engineer is about being able to identify the correct algorithms to solve your problem and being able to implement that algorithm in the language and platform that best solves the problem. Great engineers are able to build simple software and use design patterns that are language agnostic.
Once you've started to learn to identify design patterns learning a new language becomes less steep of a learning curve. Learning a new language is about learning the plumbing of the language, i.e the syntax, the libraries, and the run-time. Knowing what you *should* be building allows you to learn *how* to build it.
Exposure to different tool-sets and platform features
As you learn new languages and platforms you'll also be exposed to new tool-sets for building, debugging and testing your software. Using these different tools will help you to learn different aspects of interfacing with hardware and other software as the languages and platforms will likely have different layers of abstraction for different things. For example with C++ you'll learn better memory management. Whereas with Ruby on Rails you'll learn dependency management with tools like Bundler.
Become better at picking the right tool for the job
Learning the ins and outs of different languages and platforms will help you learn to pick the right tool for the job. You'll understand what languages excel at what types of problems. For example, you may need to write a script to parse files and find that perl's built in regular expression capabilities help you to write an efficient and simple script. You may want to build a cross platform game and decide that using C++ will help you port your game to multiple platforms without as much code duplication.