I compiled a list of some pros about Drupal being open source.
Pros:
1. Contributed modules
Like the Apple App commercials "There's an app for that," ...there normally is a module for that too. It might not be exactly what you want but it does give you multiple community options.
2. Free
I love free things, no licenses, no sign up fees, no signing up for newsletters, its just there to download and play with. Install it as many times and in as many locations as you need. There's no need to count, track, or monitor for license compliancency.
3. Easily Changeable Code
you can fix and tweak problems according to your needs. You can also learn about how the system works itself by looking at the core code. This is a huge win over closed sources where you don't even own the code.
4. Community
Lots of resources and eyes from other people. You can find help and contributions from other people, and not just one company.