I want to be able to purchase a mobile that works with all usable GSM (and LTE) frequencies. And then, purchase a SIM card that is free from all carriers but allows me to opt into specific agreements with each carrier based on the price of features.
In other words, I want a personalized mobile service experience. Not just a personalized device.
For example, if AT&T gives me the best SMS package, then I will use them for that, my SIM card gets provisioned with the SMS/MMS gateway for them only. For voice, though, I want to use the MVNO Simple Mobile, and so the same SIM is provisioned for voice only. Likewise, I can choose MaxxRoam for my international data needs, and therefore that same SIM again is provisioned for just data access.
If you will, I wan to pick and choose on a more granular level the services and service providers that serve my mobile needs.
In this scenario, having a penta-band mobile phone (hi Nokia) makes the most sense. You know, like the one next to mine in this pic.