I currently have a T-Mobile family plan with five active lines for my wife and three daughters. My wife and I have data included and even with teen daughters I think this is just about the perfect setup. I already spend time controlling how much time is spent on the computers at home and do not want to then spend time monitoring and controlling data usage across the family. I understand that shared data may work well for those of us mobile enthusiasts who have several devices, but then again you can just add tethering to your phone and connect from five to eight devices.
If carriers had launched with shared data a year or two I think it may have been a hit, but I now tend to agree with T-Mobile's take on the issue that data tailored to each user is a better option. I like the fact that I still have unlimited data on T-Mobile (yes, it might get throttled after 5GB) mainly for peace of mind since I rarely exceed 3GB (only really if I am heavily testing a new device). I was upset thinking about Verizon and AT&T killing unlimited data plans, but if I can get over the mindset of unlimited data then it isn't an issue even for heavy users like me. For the majority of people, 2GB and less plans are more than sufficient. If you use more, purchase more and get the ability to tether.
In the past I thought I should be able to share the data I already purchased, treating data the same way that family plan voice minutes are managed. Wireless carriers are not in the business of charity though, so you know that these shared data plans will likely end up costing the consumer more while also then creating another feature that has to be managed by parents and company personnel. And just like voice minutes, it will likely be that we end up paying for way more data than we ever even use.
My mind has been changed and I am now going to just focus on finding the best data and voice plan for my usage without asking for shared data any longer. I am sure there are valid reasons for shared data and many likely have scenarios where it will work for people, but it is going to cost you and we may not like the change in plan pricing structures.