The situation where you share one iTunes account for purchases between devices but each device has its own unique iCloud account for sharing contacts, calendars, etc. seems to be allowed with Family Sharing as you can specify which iTunes account to use for purchases BUT it doesn't appear to me to be the same situation as before in a subtle way.
In fact, it appears to me that all Apple has done here is separate which Apple ID has "ownership" of a purchase from the Apple ID that actually "purchased" the content. Prior to Family Sharing the Apple ID that purchased the content "owned" the content.
So the Family Sharing solution has a place but it is subtly different from the old setup where you shared an iTunes purchase Apple ID between devices. In fact, this new Family Sharing feature (if I understand it correctly) is NOT what most users that were sharing iTunes IDs really desire because now the "ownership" of the content will be spread over multiple iCloud IDs even though they were "purchased" with the same shared iTunes ID. I think most people that were sharing iTunes purchase IDs want the purchase AND ownership to belong to the same Apple ID and just "share" the purchases with other family members.
The way it seems to work though is now each family member with "own" whatever they purchase even though only one shared Apple ID will pay for the content. This will spread your purchases across all your families Apple IDs going forward!
Under Family Sharing you end up owning content even though you didn't purchase it and the organizer that purchases the content does not own it! Granted, this will only start to become a potential issue once you disconnect from your Family Sharing group but is something to be fully aware of when you go into Family Sharing (if I am correct in how this works).
Read this paragraph carefully from the following link and see if I am interpreting this correctly on what happens when you disable Family Sharing: I have not tested this so could be wrong and it will not be easy to test because of the risk of getting locked out for a year after more than 2 join attempts. Yikes!
http://support.apple.com/kb/ht201079
Under Making Purchases:
"Once paid for, a purchased item is added to the account of the family
member who initiated the purchase. In other words, the content belongs
to them, much as it would if you were buying them a book or a DVD. If
Family Sharing is ever disabled, each member will keep the purchases
they initiated, even though they were paid for by the family organizer."