Not to be argumentative, but if your family members configure their own personal iCloud accounts on the devices, you clearly don't "own and control" them, even if you have paid for them sometime in the past.
Sharing content in families has always been a gray area legally (fair use etc.). Family sharing is just an attempt to create a proper licensing framework around it. It can also potentially prevent various problems that can arise when sharing accounts (just search this forum). Apple still needs to iron out some kinks though.
My kids do not have their own iCloud IDs either. We have Messages set up to receive their messages only on their specific iPads (by reference to separate non Apple ID email addresses on the account), but they share the same iCloud Apple ID. They are only 10 and 11 years old. I clearly control their devices.
And it's largely the same thing with my wife's iPad. All 4 iPads of different flavors are also on my developer account, and I test apps on them.
Only with my wife's new iPhone did she create a separate Apple ID for iCloud (messaging and photos and contacts are more critical on the iPhone than on iPads).
But I bought the phone and the phone is in my name because I get a Verizon discount through work. In my legal opinion, I am firmly within the bounds of the license.
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The trick is that when one of my kids asks for something new that we've never bought before, free or not, I look it up on my iPad. If it looks OK, I buy it. And then I immediately cancel the download. My son or daughter can then download it on their iPad without me ever touching their device (because they can re-download previously purchased apps without intervention). This way, their device is never unlocked. And I can even approve an app purchase when we're not together.For me, the obvious advantage of Family Sharing is that instead of my kid having to hand me his phone (assuming we are even at the same location), and me typing in my Apple ID password every time he wants even a free app, I can just use Touch ID from my own phone when the request comes in.
However, I've had problems with some apps like Instagram and Swiftkey. I see they are "not available for family sharing", but what's the logic to this when they are free anyway and we can easily get around this by not using family sharing?
A second advantage to Family Sharing is I can give my wife the same approval privileges so either one of us can approve an app from our own phones.
Granted, the new approval system is slicker, but it's not worth the hassle yet.
(I'm also not sure how it handles iTunes Match music. We have a few thousand songs purchased from iTunes, but two or three times that many more ripped from CDs or purchased on Amazon and "Matched" in iTunes Match. I have read conflicting reports on whether "Matched" music will be available via family sharing. And I am not going to manage two or three or four separate music libraries.)
John