Wrong for quite a few reasons.

1) You have to be in an area with good T-Mobile coverage for the only other GSM network in the US that has coast to coast coverage. I am not in an area like that - halfway between Charlotte and Atlanta there's a lot of dead spots that T-Mobile have yet to fill up.

2) You're talking about purchasing unlocked phones. Wrong. I can do that. I can join any network that supports my frequencies - the more the merrier. T-Mobile is typically 1700/2100 and AT&T are typically 850/1900. Things like pentaband and quad-band phones - like the Nexus S or Nokia N9 - remove the guess work, but you have to have coverage.

3) When you purchase an unlocked phone, you can be contract free or under a limited contract - T-Mobile has removed their prior cooler plans - that can be PAYG or pre-paid. Those are getting harder to find - but things like Walmart's Straight Talk do help and don't matter on the airwaves.

Now, what does that all mean? Buy an unlocked phone, you will not get a lower monthly price. It is harder to locate the unlimited T-Mobile plans now it seems (search for the threads here). And there's no real monthly break for an unlocked phone.

You just can break away from AT&T and go to T-Mobile and vice versa at your own time without an ETF.