Why is the browser that ought not to exist being advertised on Stack Overflow? After all, this is a programming site, and it gives programmers the most headaches of any browser. And now we advertise it? I vote to remove it.
IE9 is definitely a major improvement over IE8.
Look at it this way: we barely support ancient IE6 at all, and I'd like to push away support of IE7 in some reasonable timeframe since it's problematic as well. The faster we can get IE folks to upgrade to IE9, the better off everyone is.
Those who have money have power. Sadly enough, Microsoft has money and Stack Exchange needs money. Advertising leads to money and Microsoft pays Stack Exchange for this.
By the way, the ad is awful.
The worst thing is that this ad shows up on my Mac. Ads are bad unless they are useful. This ad is not useful for non-IE users. It's even worse if you already are running IE9!