But your question is valid. If you FLASH a phone to Metro's network, not all modes may work because the carryover phone may not have all the chipsets necessary to work on all the bands/modes. Also, older MetroPCS phones -- say bought off eBay? -- may not run on all modes, as MetroPCS network changes were made since those phones were manufactured, like the AWS stuff. New phones bought from MetroPCS, yes.
And your question helps illuminate the LTE plan of Metro. The LTE Samsung Indulge and Craft are the two phones that will access ALL of Metro's network capability. They have the chipsets and circuitry that will seamlessly find the MetroPCS signal.
In the future, all Metro's phones will be LTE capable and will run on all parts of Metro's network.
They had 2G (1x) and were starting a 3G upgrade, but skipped that and went on to LTE. They had 3G in Dallas and Detrot, and some said also LA, but from my understanding that is gone now and it's only 2G and LTE in all markets (except Tampa which is still just 2G until the McDill AFB situation is resolved).
Metro does not want to tout Sprint is their roaming partner because people get confused and would think Metro does not have their own network, that they are just re-branded Sprint -- like Boost Mobile and Virgin Mobile and countless others (Total Call, Platinum Tel, Common Cents, Kroger's i-wirless, Trumpet Mobile, Xtreme Mobile, etc.). Those companies are MVNOs of Sprint. They are not real cell phone networks. They buy wholesale Sprint service and re-sell it under a different brand name. But it's Sprint towers, Sprint signals entirely. (Even Cricket/Leap has become a Sprint MVNO just recently, which puts them in a precarious situation, in my opinion. More on Cricket in a minute.)
In fact, Boost, Common Cents and Virgin go one step further in the MVNO land and are actually Sprint Nextel-owned companies. So, they can't really "compete" with Sprint because they ARE Sprint. Sprint is not going to let those puppet companies have better phones than the Sprint mothership brand. Think of it like the Proctor and Gamble detergents -- do you like Tide or Gain? Do you like Dawn dishwashing liquid or Joy? Doesn't matter, they are all P&G, and P&G gets your money. If one is better or cheaper, it's either in your mind or because P&G positioned it that way, crippling one product to make the other look better.
MetroPCS does not want to get lumped in with all the other Sprint MNVO's. Metro has their own CDMA network, they have their own LTE network. Metro is the 5th largest cell phone network in the USA.
As an aside, Simple Mobile is merely an MNVO of T-Mobile. PureTalk USA is merely an MNVO of AT&T.
* Cricket/Leap is in no-man's land. They are still the 7th largest cell phone network while also being an MNVO of Sprint for Sprint's nationwide 3G network. "As part of the agreement, Cricket made commitments to deliver minimum levels of revenues to Sprint. Cricket will offer products and services using Sprint's 3G network coverage throughout August 2015." See, that is getting dangerous. If they can't deliver the revenue, then Sprint will own them. Because who would buy Cricket with a multi-year financial partnership contract with Sprint hanging over their heads? Metro has been the likely merger partner with Cricket -- but not now with this Sprint contract like that. (unless the contract can be cancelled if Cricket/Leap ownership changes) If Cricket can't deliver the promised revenue, then they will have to sell themselves to Sprint to make good on the deal.
All this is why Metro is willing to have a throttle 1x roaming agreement with Sprint -- not a financial partnership -- but does not want to muddy the waters by bragging it's with Sprint.