Is this a case of taking statistics too far? To hear that only 50% of the people with web browsers can use your application doesn't sound great, never mind that 50% in real numbers is far greater than 100% of iPhones (or whatever, insert your own example here).
IE is just another desktop browser with essentially no intrinsic value over the alternatives.
That's exactly what this is about. Microsoft is the one slowing the web development even more now, by not adopting the latest features on time.
IE: 40%, Mobile: 10%, Other modern desktop browsers: 50% (Numbers based on Wikipedia's browser market share page)
Current thinking has us building a website that works for all of those browsers, and then building an optimized version that works better for the 10% of mobile users. I've visited many sites on my mobile devices that are complete rewrites, using browser-specific features, of the regular website, for 10% of the users.
Yet, we completely refuse to use features found in 50% of the browsers because Microsoft hasn't implemented them yet. This is where the math falls apart. If resources are limited to two implementations, wouldn't it make more sense to provide a better experience for 50% of the users, and still provide the basic version for Microsoft/mobile users?
If 10% of users are enough to warrant a completely different website version as has been demonstrated with the rise of mobile users, then why not build three variants? One mobile, one old web technology version, and then one using bleeding edge technologies? What is it about Microsoft's latency in feature implementations that you speak to that prevents the latter version from even being considered?
[1]: Of course, you can sell subscription on web too. It's just that far more people are willing to purchase a $5 Instapaper app on the app store (or upgrade their account for $1/month to search within their saved articles through in-app purchase, that literally requires two click (and entering your password), than there are people who would give you their credit card number to purchase your web service.