Actually I have 3 lines and we use roughly just over 1.8GB in total, but my concern is more the coverage then it is the cost. If I can not make a call or access data, there is no point in me switching anyway. I do understand there are those who will benefit greatly from T-Mobile, however most of us will not. I do not think that even if VZ and AT&T decided to do what T-Mob has with no contract, we will be any better off than we are now, they just are all trying to rub the sub-pricing off on us, so I can see them not doing reduced sub pricing and keeping the same cost for their plans, they pay a lot when they sell an iphone 5 for example, so push cost off to consumers, and they will make more $
AT&T, Verizon & Sprint have always charged full price even if you brought your own gear so there was no reason not to take the subsidized phone if you knew you wanted to stay 2 years.
While price isn't a primary concern for you, there are countless people for whom price is a significant consideration, especially those who want to move to smartphones and don't want to pay $300+ per month for a hand full of lines. I understand in your area T-Mobile may be insufficient but I'd wager there are plenty of people (in your area) who would trade mediocre coverage at times for a significantly lower bill. Also not being tied to a contract is a great benefit because if it get's too bad, you can sell your phones and move to AT&T or Verizon.
Just look at how many millions of customers are still on Sprint and their coverage and dropped calls are worse than anyone right now. And the pay more (than T-Mobile) for the service too.
Cheers,