I don't think publishing will help them out any in sales. Its a niche group of people that will more than likely get frustrated with how bad they will be "limited".
As it stands right now, anything that is sold digitally on XBL whether it be a game or DLC has to pass some strict guidelines to be approved... and modifications or bug fixes have to go through the same processes, which is why a few games on Xbox have taken long amounts of time to patch. If developers groan at these processes I can only see the outcry from regular joe blows trying to put their content out there. I don't think MS is willing to become as unsecured as the Google Play store. There is one thing I can give MS props on, and thats their crack down on cheating. It's almost non existent outside of COD map glitches. Unregulated content would be a new avenue for cheaters, They've been doing it on iPhone for years making dummy apps that are really free tethering apps to bypass AT&Ts restrictions... (This is what lead to the end of unlimited data plans FYI).
This goes hand in hand with modding... a big chunk of this target group would be modders... There will be an outcry about how people can't make their own DLC for games, Extra maps and what not. MS is going to regulate it all... and how will they deal with the achievements? Will they let these player made DLC's have extra achievements? If you can sell your content, do you get to decide a price? MS will most definitely take a chunk of your proceeds for themselves... that will be another thing that will have people crying, but what are the implications of this? Will MS change the culture of modding to where modders expect to be paid for their content after tasting the monetary value of their work? Console DLC it self has changed the once free DLC of the PC gaming world.
On a stretch, someone could make a few impressive DLC's sell them, become highly acclaimed for their work as a common gamer... and slowly build funds from these sales to start a small gaming studio... only to be purchased by EA or Activision... the end
In all seriousness, I see more PR blunders in Microsoft's future.