For many families this may work out ok. for instance....I have a grandfathered unlimited iphone. Both my wife and daughter have dumb phones. My daughter desperately wants an iphone. But we've said no due to cost. But if the pricing and data allotments are reasonable, we could save money versus just adding $30/line right now.
Sure, I would lose unlimited, but I rarely go over/near 2gb. Usually it's just downloading podcasts over cell vs. wifi. If I was limited, I would just set my podcast app to only DL over wifi. I usually do that but sometimes override the setting if I want to DL something before my trainride home.....
Again, this only works for me depending on the pricing and tiers for shared data.