The SIM is an integral part of the GSM standard and provides a huge range of benefits. The point of their existence is to establish the identity of a subscriber independently of a handset , which has a lot of interesting, useful and important side-effects. A very large proportion of GSM subscribers use dual-sim handsets, to take advantage of the cheapest available tariffs. This is the norm in price-sensitive markets…
For example, the US carrier T-Mobile has a whole lot of customers with iPhones despite not offering it themselves. That's possible because consumers are able to pick up a used GSM iPhone and stick their SIM card into it.
That isn't really happening for US Cellular, because Verizon and Sprint's (SIMless) handsets can't readily be made to connect to talk to their network.