Are there many individuals with 10 iOS devices? Families, yes. But with family scenarios, there are privacy and control issues when everyone shares the same iCloud account (ditto for iMessage and FaceTime) - ever have a spouse or kid delete your contacts when they clean up "their" contacts list, or receive your iMessages because they accidentally (or intentionally) added your email address to their "You can be reached by iMessage at" list?
I look at iCloud as a personal solution, not a group solution. Let each family member have his/her own Apple ID for iCloud/FaceTime/iMessage/Game Center, then they each have 5GB. (They can still share a common iTunes Store account.)
Thats the way its intended from the beginning and by design. Each with own iCloud email for contacts and mail, etc.