Well, aside from the obvious risks of everyone being beholden to the same OS if a security hole suddenly arises (but I'm sure that won't happen with Microsoft) there are reasons to build your skills with other OSs on your own.
Where I work I am already the Windows goto guy for light IT support because calling Bangalore is like going to the dentist only less productive.
However recently I changed groups and lo and behold, the designers use both Windows and Red Hat Enterprise. They already have a dedicated Linux support guy who is really good but it's almost a certainty that I'll be doing work in Linux before too long. The fact that I've been using it at home for years will really help me to adapt quickly.