That's it exactly. Linux offers a world of PHP, PERL and other free language capability and the programs that run on them (try finding a Windows based discussion forum-most forums for example are in PHP)
It generally has fewer OS updates depending on the version.
Windows servers are like Windows PC's, lots of patches and proprietary software, If business clients are set up on Exchange, dot-NET, and other Microsoft solutions they require Windows servers and like everything else Microsoft is constantly making their "all in one" solutions more attractive in their plan to take over the world
The next version of Office for example will contain many more features so the LA office an work on the same documents, spreadsheets, and presentations with the NY office easier than they do now---but you need those Windows servers
For independent, powerful, database driven custom apps most will use Linux for the choices of programming languages and the generally lower cost of *nix hosting (Windows gets REAL expensive if you need MSSQL licenses- on *nix MySQL is free etc, )
Some people like the all-in-one of Windows solutions for their business communications; but they probably have their own IT and servers and won't need you. A few mid-sized businesses might.
The average guy-doesn't need it unless he uses Microsoft Expressions software to design his site.