It's important to decide on OS levels as a consistant platform across a company. This is why you generally don't see companies that deploy WinXP, Vista, and W7 in an enterprise -- they'll typically wait and migrate the entire company to a singular platform for supportability reasons. It just makes sense.
In the same sense, those companies choosing to migrate to WS2008R2 are forced to use Ex2010 as their messaging solution. Since this is not yet released, it cannot be completely evaluated, plus it has the added "stigma" (be it right or wrong) of being a SP0 level product, which still scares some companies.
To the point of the original article, it SHOULD be up to the respective companies whether or not they are "taking advantage of new Windows 2008 R2 features", in the sense of Exchange 2007 server. I don't see how that is even relevent.
I was hoping to read this article and see specific technical reasons why it could not be supported.