I bought what I think Apple characterizes as an unlocked SE iPhone for about $480. Virgin Mobile has served me well enough so I went with their $50/mo service offer.
With no SIM card my phone seems unable to talk to me except to ask for a SIM card. My daughter, with developed iPhone reflexes, gets to screens that choose among WiFi services. I see no point in WiFi service in any of the interfaces I can access in a SIMless phone. I think of SIM card as an option that enhances my pocket organizer. It is frightening to think that I cannot use my iPhone out of cell coverage. I can turn off WiFi on my Mac. I would correspondingly like to turn off cell service on my iPhone. Also I cannot put the iPhone in my pocket without the screen coming on. This is a power problem or perhaps a fancier power control than I have imagined.
Apple tells me that the phone will originate and answer POTS calls over WiFi, if in range. I think that POTS service over WiFi requires the carrier to have an ‘Internet cell tower’ and handoff between towers, an old solved problem, may be harder there.