It depends on the application. For some things, it doesn't really matter if it's closed or open source (or does, but not all that much). For example, games. Game creators are mostly still stuck in 20th century business models, so they feel that they need to close their source in order to make a profit, and well, that's not ideal but it's ok, because it's a game, you just play it and that's it.
Originally Posted by BO$$
But then there's things that you definitely want to be open source. Compilers, development tools, operating systems, anything that modifies your system files, things like gparted for example. Part of the reason is trust - I would never trust a closed-source application to mess around with my partitions. But a bigger reason is, that people depend on these tools, they need to be certain that they can depend on these tools in the future; that if the current developer gets tired with the project, someone else can continue maintaining and developing it. With closed source, you're at the mercy of the software company, and if they stop supporting a tool (either because they go bankrupt or maybe because they want to periodically squeeze some more money from their customers by forcing them to a new paid version of their software) then that's it, the codebase is closed so there's nothing anyone can do about it.
And then there's things like standards. You really want standards to be open and transparent, because, well... the reason should be obvious - if it's a standard, everyone should be free to use it, no one should be required to pay for using something that is a standard, and everyone should have access to the specs, because otherwise what's the point of having it be a standard in a first place...
You would be stupid not to care whether your software is open or closed. It is indeed an important consideration for many, and dismissing those who care about the openness of their software as "zealots" is really short-sighted. Ideally, all software would be free and open source, but until we live in that world it's best to support free and open source software as much as we can. Be the change you want to see, you know.