Different tools for different jobs.
My HTPC runs Windows, this ensures full compatibility with media streaming services. Ever tried to get Netflix / SkyGo / anything Silverlight to work on OSX or Linux? I don't need that hassle!
I have a desktop that has my Visual Studio dev environment on it, I've simply not round to moving that to the Mac yet.
My MacBook Pro is my main machine, I prefer OSX to Windows (gestures, hello!). I like that I can drop to a terminal any time and use it productively. I use it for iOS and Android development, web browsing and for other general use.
I have a file server running Ubuntu. This is the central file store for the house, it sits in the back of the cupboard under the stairs with no screens attached and just does its job. Ubuntu server is suited well to this.