The way I see it, just because the majority of people do something, doesn't mean they should. I'd rather proactively change the internet (even if it's only in a very small way) than sit back and let the majority of people have their way, just because there's more of them than me.
It's not like I'm sending them a page saying "get Firefox" and then redirecting them to the FF download page. I'm just saying that I honestly don't care if IE is a piece of shit, and I'm not going to waste my time catering to people who don't have the sense (or knowledge that it exists) to get FF or any other standards compliant browser.
As spectere said, the CSS support (and other quirks) in IE6/7 are quite painful at times, and require some pretty mangled code to work, at times. I'm not going to waste my time writing code that no self-respecting coder should ever have to write, just to cater to people who aren't concerned about using a standards compliant browser. If it breaks on them, maybe they'll see the light and chose a browser that doesn't suck.
Not to mention that IE 8 fixes quite a few of those nasty issues in the IE rendering system, so by the times that comes out and is in widespread use, there'll be no need for those ridiculous hacks.