sure it costs money... its also tons of extra work taking care of 2 OSes instead of one. It also takes up tons of space on the drive... plus its Windows. Most people that buy Macs do not want to use Windows. Plus you have to reboot to play a game, making your computer useless to do anything else while playing, unless you want to switch all your software over to Windows, and not just the one game... if that was the case I wouldn't have a Mac in the first place.
There's no work taking care of Windows 7. MSE is all you need (or install Avast if you really care to) and Software Update as necessary - just like OS X.
You can put as little or as much HD space to the partition that you want. 40GB, 400 GB, or whatever you need to do what you want - just know how much space you're going to need depending on what you need when setting it up.
The vast majority of anyone that games on a Mac has Windows and OS X due to games either being only for Windows or running significantly better in Windows. Then plenty of people need virtual installations for work.
Also rebooting is a slight downside, but also a positive - it can limit how much you play when you have to reboot. I guess if you have no self control it would be the opposite. Probably most mid req games can probably be run via Parallels or VMWare, but those really intensive games need to be run natively to ensure optimal performance.