There is room enough for everyone…
I definitely lean towards open source. I am not against propriety source but at some point I feel it needs to be released to the public via open source.
I feel developing software is much more an art than a science. As an art, you have styles and influences. There is no one way of doing things but some ways can be better than others. With open source you have the opportunity to see better ways of doing things in real world applications. It can then inspire you to develop something better.
Propriety software wants to keep that knowledge to itself and with intellectual property laws, they also want to prevent you from developing anything like their software. In the end, proprietary software can stifle innovation.
Granted there is examples of proprietary software out there but open source lends itself to more innovation.
Even some big applications built as propriety software are beginning to embrace open source. Visual Studio .NET from Microsoft now bundles (and contributes to) open source tools such as jQuery. It has even released other components such as ASP.NET MVC as open source, although I believe it doesn’t take contributions, but you do have full access to the source code.
In the end, proprietary software can be okay, but open source has more potential to advance software into the future.