My work on a Drupal installation called Voic.Us has given me a window into something important.
That is, what is the most important value you get from open source?
My answer may surprise you.
It's not seeing the source, because most of us don't look at the source. It's not the price of the software, because price is a miinimal factor in making software useful.
It's the standard.
Every open source project is a standard on which members of that project can build. The more widely an open source project is adopted, the more value that standardization becomes.
This is why the value of an open source project goes up as more people use it. Whether the project is server or client based, it is the standard which provides a base everyone can build on, and everyone can benefit from.
Microsoft remains strong for the same reason, its standard. In that case, the details are kept secret, so builders are at a disadvantage, especially if Microsoft decides it wants to "embrace and extend" the standard into your niche.
But every open source project, no matter how small or niche, gets the same standards benefit as Microsoft because the code is open, and there is no owner to roll over you.