Cool web development happens on Unix. Whether that Unix is Linux, BSD, OS X, etc doesn't matter. At some level they are close enough to alike that the job gets done. Then you hop on over to some Windows derivative and things grind to a halt.
So you got some action over on side A and a sizable set of web browsers that run across all these different operating systems. And then you've got this other browser that only runs on side B.
If Microsoft wants to show a real commitment to an open web, they need to start releasing versions of IE for other operating systems. It used to be possible to get IE for Mac and Unix. There was even a time when IE was the clear best choice of browser on a Mac.
If that doesn't happen, why should I bother targeting a tool that I can't run (easily)?