Open Source on Windows: Is There a Point?

This is just something I have been mulling over in my mind. Is there
really a point to working on open source software for the Windows
platform? Does it make sense to expend time and effort building software
for a closed operating system? Would it not make more sense to develop
open source software for an open source platform?

Part of me thinks open source development on Windows is a waste of
time.The open source software being developed on Windows makes the
platform more valuable. This increased value primarily benefits
Microsoft, but does little to provide an incentive for Microsoft to open
their platform. If users can get high-quality open source software that
runs on Windows, what motivation do they have to switch to an open
source operating system? In the end, does this ultimately inhibit the
growth of open source software?

On the other hand, part of me thinks that open source development on
Windows is very valuable. If it was not for the availability of open
source software on Windows, how many users would never be exposed to the
concept? There is also the idea that using open source applications on
Windows makes it easier to switch to an open platform later. For
example, somebody using Firefox and Thunderbird on Windows would
find it easier to switch to something like Fedora Linux because they
have access to familiar applications. Without access to these
applications on Windows, the cost of changing to a fully open source
system might be too costly for some.

I do not have a final answer to these questions. As any regular readers
of this blog already know, while I make my living working with .NET and
BizTalk on the Windows platform, I also have a passion for open source
software and Linux. Some times I think it would be nice to combine the
two and develop open source tools to help with my BizTalk work. On the
other hand, I sometimes think my efforts would be better spent
developing open source software for Linux instead. I guess this is going
to require more time to figure out.