I work on open source software with two other developers and our business makes enough to pay for three full-time salaries (they're low-end software developer salaries, but we all live comfortably in the USA and Mexico).
We sell two WordPress plugins, both of which have free counterparts (think Lite/Pro). The source code is GPL for both versions.
Why do people buy the Pro version of our software? That's we've asked many times over the past few years. I feel they buy the Pro version for both the features and, more importantly, for the fact that it will be _updated_ and _maintained_, and they will have _access to support_. Site owners that want to use one of our plugins recognize that by paying for the plugin they get access to support and features not available in the Lite version, whereas the free version has limited features and only community support.
Could someone copy the source code for the Pro version and sell it? Sure. (And people have.) But can they copy our Pro support and sell that? Not so much. Can they copy our reputation? Nope.
I've learned over the past few years just how valuable "access to support" is for many people. Even if they never contact us, just knowing that we're there _in case something goes wrong_ is comforting enough for them to pay for that comfort.
If more open source projects started offering support, I bet you'd have a lot more open source developers making money.
For example, if I decided to set up `mutt` for the first time to switch over from web-based email, would I buy a "mutt support service" from someone that allowed me to submit `mutt` questions through a ticketing system while I was setting up and getting familiar with `mutt`? Yes! I would, especially if that person had a reputation in the community as someone who knows `mutt` inside out.
It's also worth noting how offering support helps drive documentation. If the documentation is great then a developer can easily point people to the relevant information and save him/herself time (so that support doesn't take up all available time). I basically used this type of "ask me a question, I'll create documentation for it and then reply with a link" method with my Independent Publisher WordPress theme project [1].
[1]: http://github.com/raamdev/independent-publisher