Whoa, hell just froze over. I wonder how the OneDrive team feels about this? This kind of a decision was made at the highest levels of the company.
Just a short while ago, anything that could be done to make their products more integrated, and increase lock-in would be done. It didn't really matter if the benefited product made money or not. (For a very visible blow, look at the Windows Phone and Windows 8.)
It's great to see that changing.
That's why some people are pleasantly surprised by this.
Box and Dropbox support office formats which makes sense why they'd partner with them to perpetuate this usage.