Excuse the complete lack of understanding. but;
Other than gaming, Why would one need to have windows on the mac?
I am going to be purchasing my first mac ever when the MBP gets refreshed and haven't seen any of my friends with macs have windows booted (not computer guru's by any standard - nor am I)
So what reasons would I want to have windows if I don't do any gaming?
This isn't a mac vs windows question to spark a debate either

I'd just like to know some of the things you guys are using windows for that you can't do on the OS X.
Regards.
The somewhat limited games available on the Mac are the main reason-- but, unfortunately, over the last two years there has been an explosion in Mac game availability. This has caused me a lot of problems with my teenagers. The former OSX lack of games is sadly diminishing.
The main advantage that Windows has is that in many corporations, Windows XP has a lock on the "corporate desktop", and, on servers, Server 2003/Exchange is dominant. Now, personally, I can't comprehend this, because if you look at Microsoft's own (every 6 months or so) security report you see that XP/Server 2003 is far more prone to malware than Vista-64/Exchange 2008 (and presumably Windows 7). However, the corporate world's bizarre love affair with XP is the main reason that people want Windows for compatibility. You would think that security would be #1 in large corporations, but, you would be wrong.
Organizations that use all the finer points of Word, Excel, and PowerPoint often discover small incompatibilities between the Mac and PC versions (some of the same incompatibilities also exist among PC versions of course). Which leads us back to specific versions of Office along with the OS.
The other main application, strangely enough, is IE. As in IE6 or IE7. For some really bizarre reason, a lot of corporate internal websites (for HR and stuff) don't work properly with IE8. Security? They think they are protected by "the firewall". Don't get me started.
I haven't had a chance to work with Windows 7 XP mode yet, but, perhaps it will allow corporations to move forward a little. I plan to try to get 64-bit Windows 7 with XP mode running under Parallels 5 soon.
So, the executive summary of my long answer to your question is, "compatibility".