could potentially be if you use enough data where you consistently go over the 1.2 TB data cap they place on their plans. If you own your own modem, the cost for unlimited data is an extra $30/month. However, when you lease their modem the total cost for unlimited data is only an extra $11/month (so $25/month for the modem PLUS unlimited data).

Of course, that's a corner case that I don't think most people find themselves in.