Well, the business upside is that they can work on features instead of debugging a broken browser.
A more esoteric upside would be that a company that speaks out visibly against IE gives me, a technical minded customer, this warm tingly feeling inside. It suggests that someone in the company is like-minded with me. Which in turn suggests that their product and direction might appeal to me, too.
It'd obviously be a different story if this was a product aiming for joe sixpack and the mass-market. But since messaging APIs are a hard sell in the joe-sixpack market in first place I can only applaud their move; focus your resources on the product, not on the latest IE bug.
