• It’s free (usually). Here I’m referring to the open source movement as a subset of the free software movement (see here).  When you’re teaching kids from a variety of socioeconomic backgrounds, free is better. You shouldn’t train kids to use programs that they can’t actually acquire at home. That’s part of why I love teaching with Google Apps – it’s free for them to get at home. Same goes for GIMP, OpenOffice, Avast, etc. If they can get it for free, they can use technology to improve their lives at home, not just at school.
• It’s just as good. Yes, proprietary versions of products can look prettier, but nine times out of ten there’s an open-source alternative that might be rough around the edges with the same functionality. Prime example: Word vs. OpenOffice or Libre Office. I don’t have Word on my school or home machine and I don’t miss it at all. Same goes for open-source virus protection, which in most cases I actually prefer to Norton.
• It’s a great philosophy. Open-source is all about freedom of information and relies on members of the community helping each other. Those are great values for any student, and I like that the story is also a concrete example of what it means to be in an intellectual community in the real world.

To me, those are the big three, but this is a nice article that highlights some of the more technical reasons to appreciate open source. Check it out.