There are some of us who are in a slightly different situation. I am currently an AT&T customer, and have been for several years. I signed up with AT&T, which was then bought by Cingular, which then (recently) became AT&T again. (Details of this process are being omitted.) I'm still using the same SIM card that I got several years ago. I'm still on the same plan (it's a better plan than I could get now). I don't want to change anything, except, maybe, my phone. But, unless I get an unlocked phone, I've been told that I have to get a new SIM card and a new plan, because current AT&T phones are not compatible with my old SIM card (being locked), and going to a new SIM card necessitates going to a new plan. I'm not happy about this. I don't want to switch providers. I want to continue to be an AT&T customer. I just want to be able to keep the service they've been giving me plus an iPhone. No deal. Not even making it a 2 year contract with the current service (plus data, of course).
So, the only way that I can keep my current service plan is to use an unlocked phone. (It's what I'm doing now. When the phone that I bought with the plan died, I bought a new, unlocked phone off of eBay.)