So you think people are going to carry their expensive iPhones everywhere? In some places, it really would be best to switch SIMs, unless you'll be staying in clean, safe urban areas most of the time. But that's not always the case for some.
How about these examples:
1. This isn't good work phone for blue collar workers(and yes some CAN afford an iPhone, some make decent money), since they are doing physical stuff a lot. But if they can switch SIMs, they can use the iPhone on their off hours if they want. You can see why blue collar workers use clunky Moto Nextels most of the time on the job, you drop those and they're still good to go! Drop an iPhone, start crying!
2. And even if you're a white collar worker, suppose you're required to use a PDA on the job but still want to use the iPhone after hours? No SIM switching sucks.
3. I suspect some parents will spoil their kids and send them to school with an iPhone, but that is such a BAD idea. If you do get your kid an iPhone, you really should have them switch to a cheaper phone for school to prevent theft and they can use the iPhone after school. I know it's not cool, but iPods get stolen at school a lot so what's going to happen when iPhones show up?
4. What if you do end up having to go to less than comfortable areas, of whatever type? You need a phone but the iPhone could get easily damaged, stolen, confiscated or whatever. Time to switch SIMs. Unable to do so, then you'll have to leave the iPhone home or risk losing it.
Just some of the reasons people would NEED to switch SIMs.