Basically my thinking is that it gives indies another storefront to stand out and sell their titles having broad reach as long as the potential customer base does in fact move over. In theory (and if it follows iOS, etc) give indies and smaller studios help in recognition either by being featured on the store, and having one place to look rather than people having to surf the web looking for a game they want to play. It doesn't solve the issues in marketing, branding, etc... but it does make it ten times easier to find games and applications having them all in one place (for example when they added the MacOS App Store I found a bunch of small tools and apps that I would have been using all along but I had no idea they even existed!)
Regarding the submission fee I am not in full agreement, but again I could be wrong. Of course people don't like to have the submission paywalls (i.e. Mac, iOS, Steam) but if someone builds a game that is polished enough to actually be worth selling they can probably come up with the submission fee. I know I am generalizing/guessing on that point but that was also a something I read a lot regarding the iOS app store when it launched but I really haven't heard it since then.
I agree with you there, the only reason I boot into Win7 is to play games or work on a port The sole reason I'll be picking up Windows 8 (at some point in the future) is for porting our title to metro.
Finally, this is a board where a significant fraction don't even target Windows, much less a specific version of Windows.