Having now (well, soon) 3 different SIM cards sizes available must be a real pain for carriers as it is for consumers, but at least the smaller cards are compatible with adapters that allow you to use the SIM in other devices. Since the microSIM came out, I made a point of always getting a microSIM for any new SIM I got as I can always use an adapter to use it in a device which needs a larger SIM, but can't use a larger SIM in a device designed for a smaller SIM. I will be changing my SIMs to nano SIM as soon as I can, regardless of device for this reason.
Back to the point however about sim-less phones. I really, really hope that we don't go down this path. Firstly, I love that I have the ability to swap my SIM between phones on a whim. If my phone went flat, I can simply put my SIM in another phone which had battery and continue to use my services. Likewise, when overseas, I am able to swap the SIM over to a local SIM if I choose to save on roaming costs.
Even if the carriers allow you to move your sim-less services between devices easily, I still think this is a BAD idea. When I roam, I often take my local SIM out and put it in a dumb old Nokia I have. The Nokia has a fantastic battery life, and no data capabilities - so I have no need to worry about huge data bills, but can still have my local number roaming to make or receive calls or SMS as I need.
If I had a sim-less device, it would be impossible for me to do the above, as I may only be able to swap the number over to another SIM-less device. It also allows the carriers to have greater control of how and where I can use my number.
If I need to contact them to move my services between devices this will be very painful, even with self service options from the carrier.
I realise that SIM-less devices are speculation at the moment - but I am keen to hear others take on them.