Microsoft cannot keep up with everything in the computer world. There are just too many forums and websites that deal with computers. Posting to those websites and forums doesn't guarantee that Microsoft will see our wants and needs.
I'm not sure that's entirely true. I believe that MS, and any savvy corporation, can and should keep track of EVERY site that mentions their name, much less one that holds full-out discussions about it's product(s). Sure, it's alot of resource to throw out there, but the corp needs to continue making $$. The only way to accurately get feedback is to hear the customer at the street level. What are they telling their neighbors about the corp's product? etc... It just happens that our "street" is electronic and YOU are my neighbor.
As far as participation in these forums, I'm glad that MS doesn't officially have a presence here (or in Seven Forums, etc...). If I want to talk to them directly, they have a web interface. The difference is, the people here help because they WANT to be here. If MS made an appearance here, the person on the other end of that connection would be here because they would HAVE to be. It changes the quality of the answers and opinions, in my view.