Unless, you actually interrupt the contract with AT&T, but in that case it's not much different than it is today, and you just pay the ETF and you're on your merry way.
Get a $200 iphone5 from AT&T, next day unlock it without paying ETF and get a cheap prepaid T-Mobile sim and never use the AT&T sim again; thus not paying AT&T's voice/data fees(and we all know they get you on overages). That'd be a problem for the current business model. To make up for the lack of overage-charges they'd have to make the phones more expensive or raise the monthly minimum charge.
NOTE: I understand the awkward issue of travel though. I made the mistake of using my AT&T sim in Nigeria once. Maybe smartphones should accept difference SIMs after it determines its in a country other than where it was bought. But of course, then I go to amazon.co.uk and order an iPhone5 to be sent to America. ;)
Huh? You would still have to pay your monthly contract to AT&T even if you no longer used the AT&T sim.
Caveat Emptor and everything, but routinely exploiting ignorant customers should not be a business plan.
There's no issue with unlocking your phone and using T-Mobile while continuing to pay your AT&T bill (except to your wallet). Nor is that scenario outlandish - just replace T-Mobile with a foreign carrier while traveling.