What would be the use? Some people are so unimaginative.
Imagine that someone traveling on business abroad, which is often the case with me, has one number that works just the same, so his daughter or wife can call and get him any time they want, by dialing the same number. The same would of course be invaluable for other friends and business associates.
Not to mention that most people could drop down to the minimum minutes on their phone plan with a service like this.
Also, imagine that someone has put a data only sim in their iPhone, from say some other device that has an unlimited data plan. Someone could hypothetically cancel their AT&T service all together and have a full service iPhone for the price of the data only sim.