Heck even with a 'phone' it's useful. Imagine you arrive in Hong Kong at midnight and you want to move your phone to Vodaphone. You don't have to seek out some store and buy a SIM - Just happens presto.
When I travel with my phone, I don't even want to turn it on before I put in a new SIM for the local system. Turn it on, it registers with the local carrier and your home carrier starts forwarding calls to it -- at international rates.
I certainly don't want "presto" reprogramming my SIM. I don't want to have to call my home carrier to tell them to move it to X, and then X to have them move it back, and have one or both of them charge me for the privilege of screwing it up so I have no working phone at all. No thanks. That's one of the benefits of having GSM versus whatever. The phone is the SIM, and I can carry more than one to be more than one thing. And I can use the second SIM in my backup phone without it costing me a second plan on both carriers.