The combined market share for, say, Firefox 3 and Safari 3 is larger than the overall market share for Mac OS X. Plenty of developers write desktop software that only works on the Mac — why aren’t more people writing apps web apps that only work in truly modern web browsers?
I hear web developers and designers complain about spending 80% of their time fixing bugs in Internet Explorer. Imagine how much cleaner and spectacular their work could be if that time was spent polishing the app itself? At the least they’d be more productive without having to devote so much time to the garbage browser. I don’t think this would work in all cases right now – I don’t, for example, see a major financial institution ignoring IE in a sight rebuild/redesign. But for new web applications delivering innovative ideas and functionality, why not?
I’d be interested to see what the browser usage is among early adopters of web applications. My guess is many of them aren’t using IE.