Very few phones work on both CDMA2000 networks (Verizon and Sprint) and GSM networks (AT&T and T-Mobile), and they're hard to find in U.S. stores.
If true (I don't keep track of which phones are available through whom and do what), I don't see what bearing that has on whether it's a good idea or a bad one to make it easier to force carriers to unlock the handsets that they sell. Even if there were *no* dual-network phones, you'd be able to move between a choice between two carriers (and the MVNOs of each), and that's better than being forced to buy a new phone.
Mail order doesn't let you hold the phone and get a feel for its size, weight, screen, and buttons before you buy.
Well, I can't argue with that; if you don't have physical access to a device, then you can't judge it first-hand based on its physical attributes. In my case, I went to the store of my chosen carrier, tried out their phones in the store, and bought a variant online that the store didn't carry. Having the opportunity to handle a phone before buying it varies on a case-by-case basis so much that I'm not quite sure why you mentioned it.