Wow, I love how people expect everything to be instant nowadays.
"WAAAHHHHH I NEED TO WAIT 3 MINUTES TO START A GAME!!! WAAAHHH

"
It's not just 3 minutes, and it's not just TIME. It's time, effort and hassle, and losing access to what you had been doing before the game. And it's not just once, it's every time you game. And that DOES take away both fun and productivity--and can even make you game less often.
You may not be thinking this through completely--rebooting works fine for you and for many others, and I won't deny that--but not everyone has the same needs and habits you do.
I do understand why some people choose to reboot and deal with Windows. But here's why, for me, that is unacceptable:
1. I want to have my same browser, bookmarks, email, chat apps, text editors, and other tools at my fingertips. So when it's time to research a game question, gather other people for a game, download maps/mods/patches, or other activity AROUND gaming, I'm still comfortably at home in OS X.
2. Rebooting means shutting down all the "stuff" I have open on my Mac, and then "getting it all out again" after the game. (Most Mac apps play well with others: you can leave tons of stuff open without a performance problem.) People complain about having to insert a disc to play a game--and rebooting is FAR more hassle than that. I often have documents spread out in different spaces, web pages open, emails open--all kinds of windows that I’m using together for a certain task or project. If I close them all, not only do I have to manually open (and even re-position) them all again after gaming, but I have to REMEMBER all the things I had open in the first place! It’s like having to move all the stuff off of your office desk, but it away in cabinets, and then remember where it all came from to get it back again later. It’s not just a time issue, it’s a productivity issue for me. Some people don’t do a lot of complex multi-app work, but the Mac at excels at that kind of productivity, and I do--all the time. And I like to mix it with gaming.
3. I will game MORE OFTEN if I can quickly and painlessly launch the game, play for a bit, and then get back to other things. Rebooting (twice) means delay and hassle, and I simply won't play as often if I have to face that.
4. Since I use my Mac as a DVR (via EyeTV) to automatically record HD TV shows, rebooting would be a problem: the DVR software wouldn't be running, and if a show came on while I was gaming I would miss the recording. This way I don’t have to check the TV schedule before gaming!
5. The general hassle of dealing with Windows. Gaming should be fun, and battling Windows is not fun for me. OS X is stable, secure, and much easier to troubleshoot if something goes wrong. Games are already the LEAST stable class of commercial apps... the last thing I need is to be fighting Windows as well!
6. Security of the Mac side: unless you stay off the Internet, running Windows can THEORETICALLY open your entire Mac hard disk up to attack. This has never happened to my knowledge, but there are at least two potential risks to even the Mac side:
* Windows can't see the Mac partition, but a virus/worm/other malware can still see the physical drive device--and theoretically could erase the whole drive, Mac and Windows alike.
* Software exists for Windows to let it read/write on Mac partitions. A virus could theoretically include such software, allowing it to mine or attack your Mac partition just the way Windows users have been attacked.
Again, that's theoretical--but still enough to give me pause. I'd rather not have to think about it. So I like VMWare. If you run Parallels or VMWare, games may not run, but at least your Mac stuff is safe!
7. Security of the Windows side. Aside from attacks coming from Windows and reaching the Mac side (highly unlikely) there is the issue of Windows attacks reaching the Windows side! Yes, you can get a virus in Parallels, VMWare OR Boot Camp, if you do something careless. There's nothing to stop that other than what every PC user does (or should do). Which means run a bunch of anti-malware apps that slow your system down and are a major pain each time you start up. And which work great--until they don't. So you have to keep yourself educated--which is additional aggravation. I don’t want to spend time keeping my knowledge and my machine up-to-date on malware defense for Windows. See above under “not fun.” Many Windows users stay secure--but they have invested in learning how, and much as they take it for granted, it IS a chore until you’re used to just accepting it.
8. Disk space. Setting aside a partition for Windows shrinks the storage space I have left for everything else.
It’s a personal call--game on Windows or stay in OS X--and neither choice is wrong.