Unlimited data plans are back. But are they needed?

The overwhelming majority of customers on AT&T, Sprint, T-Mobile and Verizon don't even use 2GB per month, which is below the data caps for all but the stingiest plans from all carriers for the common two-year contracts. And virtually no one uses over 3GB. T-Mobile, however, does have the most data-hungry users, according to NPD.

Eleven percent of customers use more than 3GB per month, versus 4 percent for AT&T and Sprint and 3 percent for Verizon. "T-Mobile consumers tend to be younger," said Hold. "Younger demographics are the ones we find hit the network harder by doing a little video and music streaming." And even when customers have had unlimited data at their fingertips, as on Sprint, most didn't take advantage of it. "In general, Sprint's usage is the same as the other networks and less than T-Mobile," Hold said. One reason people stay under their limits is that they often use their smartphones over Wi-Fi. And even if people use only the cellular network, they will probably be fine. For example, 2GB per month is enough to do all the basics, such as email, Web surfing and Facebook posting, as well as streaming music for 30 minutes per day.