Microsoft Internet Explorer 11.0: 2.57%
Microsoft Internet Explorer 10.0: 32.63%
Microsoft Internet Explorer 9.0 : 16.34%
Microsoft Internet Explorer 8.0 : 37.48%
Microsoft Internet Explorer 7.0 : 2.26%
Microsoft Internet Explorer 6.0 : 8.49%
Microsoft Internet Explorer 4.0 : 0.23%
(Source: NetMarketShare. Note that this is global so skewed a bit by all the pirated copies of Windows XP in China, etc)
IE is a bit of a mess browser-wise because we've nearly always had a ton of users not on the latest version. The reason? IE doesn't automatically update to a later version. Every other browser on Windows and Linux does. The only other exception is Safari on Mac OS X, which has specific versions artificially pegged to the OS like IE is with Windows. That's why we have 38% of IE users on an outdated browser like IE8 and 8.5% on IE6. Like IE8, IE9 is lacking in many important areas that users of Firefox, Google Chrome, etc don't need to worry about. IE9 lacks several CSS3 tags that have been supported for a while by Firefox, Chrome, and Safari. IE9 lacks columns support, animation support, transform, and transition support. IE10 and IE11 properly support it.
In short, Google can keep making Google Apps better by dropping support for dead browsers like IE9. I mean dead as in they're not being developed further. They're stuck with broken or non-existent CSS3 support. And, as pointed out by others, the type of organization that's going to try out Google Apps as a MS Office replacement, isn't the type of organization that's going to be stuck on IE8/9.