It's not because of the time it takes to reboot.
People usually enjoy idling in MMOs, even using them as instant messengers and so on. Because of this, many people, like me, have their games running in the background while doing other stuff. Or they might want to splitscreen play/chat on desktop, or whatever. Endless possibilities.
Now, if you have to use Windows everytime you want to play GW2, you have to stop doing whatever else you're doing at that time, or install everything (have fun with constant Skype synchronisations) on both OS. While that's of course all possible, it's really annoying.
I'll give you a small example from me. I'm an iOS developer, so I usually have my coding environment active at any time even after work, casually switching between programming and playing/chatting and so on. Now, a friend comes online and asks me to do an event with him real quick. I have to save and close down everything I'm currently doing, reboot, start GW2, and play. Say, my friend goes afk for a few minutes. Instead of using the time to read through my source code, I have to do something else, just because you can't develop (legally and/or easily) for iOS on Windows, at least the environment is drastically different. Usually someone might IM me while I play and ask me for some information, which I can't access from Windows (maybe because the password is stored in the Mac Keychain or whatever). Now I have to ask him to wait until I stop playing, which plainly sucks. After I stop playing, I have to reboot again into MacOS, open everything I was working on and proceed. Obviously that's much harder than just alt-tabbing around.
You see, while it's of course not impossible to just boot around, it's a real pain.