I think that most Linux users are beyond the mantra that Linux is for everyone, but there are a lot of situations where Linux is just as good as Windows. In those cases, choosing the operating system first may be appropriate.

Let's face it: you can get good software in many categories for Linux. LibreOffice may not be Microsoft Office, but it certainly performs the task beyond many people's capabilities. The same can be said for graphics design. It may not be professional grade, but the tools still perform the task beyond the capabilities of most users. Of course, Linux is also on par with Windows for many software categories. That ranges from general use (e.g. web browsers) to specialized applications (e.g. software development).

Choosing the OS first in those cases may make sense for some users. If cost is a constraint, you don't have to worry about buying a license for Linux or most of the software that runs under Linux. Put another way, you don't feel compelled to buy Photoshop (or even Photoshop Elements) just because of its branding. You don't feel compelled to buy Microsoft Office just because of its branding. You don't feel compelled to buy Visual Studio just because of its branding. License restrictions are also another thing that you don't have to fret over. You can legally run Linux, LibreOffice, GIMP, Eclipse, etc. on every computer that you own. You can also do so without a second thought. You also don't have to be concerned with the restrictions placed on the use of cheaper home editions of software, such as Microsoft Office or Visual Studio Express.

Now I'm not saying that choosing the OS first is appropriate for all users. If you're doing something specialized, it's a bad way to go. If you're doing something that is well supported on all platforms though, it may be a better starting point. I'm also not suggesting that choosing Linux is the aforegone conclusion of choosing an OS first. You may choose Windows because it makes hardware selection easier, and supports popular FLOSS anyhow. You may choose OS X because it supports popular commercial software and provides a competent Unix environment.