Honestly, much as I howl about IE, I can't expect people to switch. Especially by making a site that doesn't work on IE.
Many of the people who use IE are people who just double click on the nice friendly icon on their desktop that says "Internet". If your design doesn't work on IE, these people won't blame Microsoft, they'll blame YOU. After all, every other website out there works on IE. Most people have no concept of a W3C standard, and figure that the WWW should just use the MS standards since they have the largest market share. Sure, these people need to be educated, but making a website that doesn't work on IE won't solve the problem.
For the rest, there are actually some sorta real reasons to use IE - even version 6. For example, if you're an IT person for a company with more than a dozen or so computers, my understanding is that there are very easy ways of pushing updates out to all the computers in the company at once. Firefox has something like this, but to a busy IT person, why do the extra work? This was the case in the place where my wife works, until she and whole bunch of other people in the company started howling about IE until the IT guys listened. (one of the IT guys is also her father which helps) They now have Firefox. Does Opera or the upcoming Safari have any corporate network friendly features?
As somebody else brought up, IE7 isn't available to everyone using Windows either. For the computers in my family, we have 3 computers with Windows on one of the partitions. I bought them off of ebay, and they are not valid copies of Windows. So no IE7 for me. Oh, my wife's computer has Windows on it too, but I haven't checked if I can put IE7 on there since she always uses the Ubuntu side. Hasn't booted Windows since she bought it.