Re: A side-effect
making their devices with an assortment of a few bands selected at random and, if necessary, making six different versions that are tailored to the frequencies used by one particular provider. The result of this is that it is hard to move devices on to or off of a provider whose band isn't one of the most typical in use.
While the number of LTE bands grows, 2G bands have been dropped, and 3G bands are imminently to be dropped, allowing some bands to be dropped... OR reused meaning they can stop using other newer bands.
Your experience with handsets is not mine at all. ATT and T-Mobile phones have each other's bands quite well covered (if not completely). The latest new bands the provider is pushing may lag behind in phones from the other carrier, but they get there. If nothing else, carriers want their phones to be able to roam onto the networks of others, where necessary.
As for Verizon and Sprint, their 2G and 3G radios are completely incompatible with everything, anyhow, so they're a bit of a walled-garden. Give it a couple years, once 3G CDMA is shut-down and they'll start looking pretty standard and interoperable, too. They already have the main bands because, again, they want to be able to charge their customers lots of cash for time spent in roaming areas.