My last employer developed on Windows and deployed on Solaris (trading software in C++). Chief reason for this was, as anyone who has cross-platform experience will tell you, Visual Studio is a world-class environment. It really beat anything available on Solaris (yes, including Emacs) hands-down. Second reason was that developing cross-platform forces you to keep the codebase clean - when a customer asked for an AIX port, it was easy. If we were a pure Solaris shop, we'd have struggled to know what was clean and what was actually Solaris specific without us realizing.

The wrong reason to do this is just 'cos MacBooks are more fashionable than Dells...