The wonderful thing about most open source projects is that you have a chance to contribute to it, either through coding, bug reports, testing...or just using it, and letting the people who wrote it know that you appreciate their work. And, if you want to modify it, you can, since you have the source....or if you just want to see how they do something.

As well, there is also the paranoia of the US NSA having added backdoors to Windows letting them spy on you and your work. With open source, you can actually look at the code to make sure the NSA isn't spying on you.