Apple loves to use and impose new standards for their products. That's why you need a micro SIM in order to use an iPad 3G or the iPhone 4.
If like me you travel a lot and get SIM cards in various countries to use in your unlocked iPhone 4, you're going to need a SIM card cutter, as most carriers aren't offering micro SIMs, especially for pay-as-you-go plans.
Sure you could do this yourself, but do you really want to take the risk of messing up your SIM card? If not, then a SIM card cutter is your best bet...