There's nothing inherently difficult running mixed-OS systems as such. Wine, VMWare, and multiple servers are all feasible for this; there may be an extra layer of debugging involved, but debugging such problems as I've encountered them has been far easier than debugging my core code

Most high-volume, automated trading shops run mixed Windows/Unix/Linux systems, with Linux/Unix as their core OS. It's been that way for years and continues today -- just take a look at the job postings.
People will tend to use what's best for them and what they know, as IAS_LLC has apparently done. I don't see any reason for this previously-useful thread to degenerate into ad hominem bickering.