This is reassuring. Still don't like the partnership if it's going to create exclusive content though. But if MS really is going to let me use whatever VR hardware I want then that is great. And especially if they really are helping others develop the tech.
I think it's more about ecosystems. Microsoft wants to get its Windows Mixed Reality platform onto as many HMDs as possible, while Facebook simply... wants more people to try VR. Microsoft and Facebook have a lot to offer each other atm, and they have collaborated on helping each other's ecosystems out.
Instagram, Facebook and Messenger apps for Windows 10 Mobile / UWP, for example. Facebook had no reason to do that, unless MS was offering to help them out with their ecosystem woes, the biggest of which right now is undoubtedly VR.