The key benefit of open source is transparency. (That's a transparent Mozambique garnet, from CLDJewelry in Tucson, Arizona. Transparency doesn't have to be perfect to be beautiful.)
The key benefit is not that the software is free. It's not that you can edit it. It has nothing to do with the obligations of the General Public License. It's inherent in every open source license out there.
The key advantage of open source is you can see the code. You can see how it works. You can take it apart. You can fix it. You can improve it. Most people do none of these things, but all benefit from this transparency.
The benefit became clear when I got responses to a ZDNet post called Is Linux Becoming Windows? The news hook was a Peter Galli story about how some folks were getting upset over the feature bloat now taking place in the Linux 2.6 kernel.
Those who responded said simply that the complainents, and I, had lost our minds. Kernel features aren't mandatory. Just because something is supported doesn't mean you have to do it. You can pick and choose among features, because you can see the whole code base -- it's transparent. You can look at the various builds out there and, if there's something you don't like, something you can do better, you can fork it, and maintain your own enhanced code base.
When Microsoft changes its software it makes things incompatible. When Linux software changes this doesn't happen, because the change is transparent. New builds are transparent, and if you come to a fork in your operating system road you can take it.
Transparency is the key term. And it doesn't just apply to software:
The same point is clear in software as in business and in politics. Transparency wins.
This is the great lesson of the 20th century. All the more opaque systems -- fascism, communism, dictatorships military and religious -- failed the great test against more open, transparent systems of capitalism and democracy.
The triumph of open source, then, is simply the lessons of history applied to software.