Well obviously MS thinks there is a good reason since it has:
1) Developed IE for the Mac - which they cancelled a long time ago and never was well supported compared with its Windows equivalent (IE histoically was heavilly tied into Windows, but that could not happen on the Mac)
2) Developed a version of WMP for the Mac which never worked well, and was generally regarded as crappy and again, never supported the same feature set as it's Windows cousin due to how tightly WMP is tied to Windows. Even today third parties can only offer limited playback of the Windows Media formats.
3) A messaging client that is less feature rich than on Windows
4) Office suite that offers versions of Work Powerpoint and Excel that are very different depending on what platform (Mac or PC) you use. And we are talking about things like document fidelity here not to mention feature set. They don't even offer Outlook but some second rate clone called Entourage that doesn't even support Windows formats (like outlook PST files) probbaly to keep you locked into Windows in some fashion.
Microsoft commercially develops software for Mac because there is a market for them to do that (Office for Mac reportedly sells tons at a high margin because its not sold under volume licensing like Windows office usually is) - thats how business operate - but there is a major conflict - they don't want them to become viable contenders with their Windows equivalents. MS has always treated cross platform as a lower priority so that nobody gets any funny ideas about leaving the Windows camp. If you think about it, it kind of makes sense - most people who run boot camp or a VM solution are running Windows in some fashion and one of things that I always see on a Windows VM is Office - the Windows office simply because they need things that can't be done on the Mac Office. Now obviously thats Microsoft's choice as a business, which they are free to do (as long as they stay out of the anti-competitive club that they tried to join back in the day).
When Microsoft says they support interoperability, I have as much faith in them as when they say that IE will support "open standards". Sure, they may do that, but I need a lot of proof.
