Microsoft's biggest problem when entering the tablet market is the sad state of their ecosystem. And to try to fix that, they decided to sacrifice the desktop user experience.
I think that's a pretty bold move. It's also a really really stupid move.
Why stupid? I think this demarcates where Android went wrong. Google allowed the tablet OS to ship and run all it's phone apps really badly. That set a terrible precedent and left people wallowing around in a sea of unoptimized apps (the oft repeated claim that Android has no tablet optimized apps is as much that people can't find them as that they don't exist). I think they would have been much better off drawing a line in the sand and promoting explicitly tablet optimized apps and relegating unoptimized ones to a clear secondary status.
By contrast, MS is saying, if you want prime real estate for your app in Win8 then it's going to be Metro and hence tablet optimized by default. As a result I am sure that every major software provider is going to ship metro versions of their apps, if only to get themselves onto that metro UI that every user is forced to see by default and into the store in a priority position.
They're throwing away the market domination they already have (to some extent at least), which of course is the backbone of their business, to try to get the money to pay the entrance fee in a market dominated by others, as a late comer with slim chances.