You don't have to wait for Family Sharing for this purpose. You can use more than one Apple ID on an iOS device (or Mac) right now - there are separate sign-ins for iCloud, iMessage/FaceTime, iTunes & App Store, and Game Center. So, if you wanted to keep your iCloud contacts and calendars and iMessage/FaceTime info/identities separate, but want to keep sharing your iTunes & App Store purchases, you can:
Create an Apple ID for each family member that they can use for iCloud, iMessage/FaceTime, and Game Center sign-ins, and leave the existing Apple ID signed in at the iTunes & App Store.
You do NOT have to create a new iTunes & App Store account for each Apple ID.
This has another benefit in addition to privacy/segregation of personal data - it means each family member gets 5GB of free iCloud storage - sharing and managing iCloud backup storage and Photo Stream can be quite a challenge when several people are sharing it.
The new family sharing feature is not required to do this - people have been doing this for years.
What the new feature simplifies is maintaining separate purchase accounts for each family member, in order to keep track of each person's purchase activity (including that parental permission pop-up). It also means that the current "error" of creating and using a new iTunes Store account when you really wanted to keep sharing the existing account can be fixed. Apple will not merge Apple ID/iTunes Store accounts, but this new feature is the functional equivalent.