Actually, thanks to market forces and monopolies of various sorts, you are eventually forced to use proprietary software. The example someone else gave was Microsoft Office file formats.

Here's a better example: almost all webdesign, graphic design, and other jobs of that type want you to have experience in Adobe products. So what ends up happening is that because people want the jobs, they learn Photoshop. Now it may be a great program, but it's still proprietary and you get locked into Adobe's upgrade cycle because the latest version is The Industry Standard...always, it's fucked up how that works really.

So if someone has had experience with the GIMP or some other free software, and can do the work just as well as a Photoshop user, their resume will be dumped because the Adobe name has a lot of weight. That someone will then figure out that to get a job, they'll also have to use proprietary software.

You don't live in a vacuum where you can do what you want.