If you're not leaving the country, what's the point?
I used to be with at&t and had no issues getting me phones unlocked once my contract was over. Then I used my phones (2 x iPhone 4, iPhone 3G and iPhone 1, 4 lines total) on t-mobile.
But that's where compatibility ends. You can't use your iPhones on Sprint or Verizon, at least not on their Edge/3G network and I believe the LTE bands are also different between them, but I haven't tried. I also see no point of taking my phones to StraightTalk or such. Their service is not as good a t-mobile's (at least in my area) and they're more expensive.
So I don't see this as a big deal, unless you're planning to go overseas with your phone. And even then you'll have issues with CDMA devices (Sprint and Verizon) and not everybody uses the same frequencies as at&t and t-mobile do here. Unfortunately, there is no "world phone" in the current smart phone playground.