The discussion is around web design / development - a field with an extremely high number of people running OSX (I'd say that the majority run OSX, but I don't have any statistics to back that up, just observation).
No, the discussion is around web design clients, the majority of whom in my experience run IE6 on Windows XP at 800x600 with the brightness turned up to 11.
Working on calibrated equipment is about having a neutral, uniform reference so that you can actually see what you're doing without having to second guess yourself. You can draw an analogy to reference monitors for sound mixing. Monitors have two properties: they reveal detail in the sound and they have a flat frequency response. The former is obviously important, and it's why as a designer you should have a nice monitor. The latter is important so that when you work in different locations, you don't have to recalibrate your expectations. There may also be non-linear perceptual illusions associated with certain aspects of the signal being attenuated or boosted, which would be another reason to have a calibrated/flat reference, but I'm not certain if this would be relevant at the scale we're talking about (between calibrated and the whole space of non-calibrated color profiles).
There are color management solutions for Windows and Linux as well -- the important part is that you calibrate your display (or other output device) to create an ICC profile, and that there's a way to install the ICC profile. (Example: http://www.lightroomforums.net/showthread.php?14070)
It's true that ICC profiles don't get you much if everyone involved doesn't have a calibrated display. And many users don't. That said, it's still valuable as a baseline and when working with other designers.
http://en.wikipedia.org/wiki/ColorSync
In comparison, the color handling/prioritization in Windows and Linux is crap.