Great post!! Personally I think having Steemit.com as the "catch all" platform is a good idea. When all of these are released and working. I see Steemit becoming the desktop client that people will go to and Zappl, Busy as mobile apps. Each will find their own niche.
It's almost like Facebook launching back in the day with Instagram and Whatsapp alongside. If they did it that way you bet they would consolidate all the data feeds.
The key thing to keep separate is branding and marketing. These platforms will eventually create their own following and personas and it will be important to appeal to each.