Sure, you're not going to be a professional software engineer working on complex problems and scaling servers at Google, but you can learn a little something from their service. For beginners who have never programmed I think its great because you can program right in the browser; no need to install compilers/interpreters, text editors/IDEs, DBMSs or servers. These things can take well over an hour to install, configure, and learn how to navigate through. All of this getting thrown at a beginners face would do nothing but discourage them.

And even if you're a well experienced programmer, you can use Codecademy to get your feet wet in a new language. I don't see the harm in this.