The article mentions that one of the advantages is that you get to work in a team environment with really smart people. This is important to me - no matter what your day job, if you're dealing with open source projects, you're dealing with people who really want to be developing code. Working with people who love what they do can be really inspiring for you. If you've been tiring of the software industry, working with enthusiasts might reignite your interest.
For some people, getting into the code can be difficult, but writing documentation (articles, tutorials, FAQs) is just as important to any open source project. After all, just having code isn't going to get people interesting in using the project in question. Writing about these projects is something that I really would encourage. Most of my own contributions to open source projects (e.g. ECF, Riena) has been through articles I've written about them rather than any code contributions. If you're looking for the platform to write on, why not use JavaLobby, .NET Zone or EclipseZone?
But, what I'd like to know here is what motivates you to become part of an open source community. If your reason isn't on the poll list, just leave a comment in the section below. I'm sure anyone running an open source project is looking for people to contribute either code or documentation. Let us know in the comments if you're looking for people to help out.