The main reason to use Windows for web development these days is customer demand. It may suck in many ways as a web dev platform compared to OS X/Linux, but if you have a stable of legacy/enterprise clients who need to "stick with" Windows for some reason (IIS servers on the client side, backend integrations that would be costly and time-consuming to replace, etc), switching to an all-Mac environment isn't going to make your life any easier.
Of course, it's possible to develop for Windows from Mac machines, and vice versa… Again, this is just opinion, but using a Mac laptop/desktop with a virtual machine Windows environment is much easier long term than attempting the reverse (and actually, you'll want at least one setup like this to validate development on Windows browsers anyway).
No platform is perfect, but to use an overwhelming generalization without backing it up with facts and figures, the main drawback with the Mac platform for web development is the upfront cost, as you say. And even that will often gain you some productivity or other ROI wins down the line.