A cap on the amount of shared data each person in the plan is allowed to use should be adjustable at any point in the billing cycle. Good Use Case: my kids are on my family plan, and I want to limit their data usage. They always use more than my wife and I.
Another possible use case, from a small business perspective, if you add a few lines on your plan, then you may allocate to specific type of employees, such as sales reps., specific amounts of data. There are several types of widgets can be used, such as a pie chart, and the total pie represents the total data package, and each slice represents an allocation to each member of the shared plan.
Update: I stand corrected, and do see an AT&T Smart Limits for 4.99 USD per month: