1. I have Wi-Fi at home. I have Wi-Fi at work. I have Wi-Fi at school. I have Wi-Fi at church. 90% of my clients have Wi-Fi. The vast majority of the time I'm somewhere where there is no Wi-Fi, I'm probably driving. As such, a data plan for me is an expense that is largely unnecessary. I'm not in a position where it's *that* vitally important that I respond to an e-mail within 30 seconds, and if someone wants to communicate with me that bad, they can -gasp- call me or...wait for it...text me. Spending twice the price of a 1,000 minute T-Mobile plan for 600 minutes and data just isn't worth it to me. Heck, I could throw in an EDGE plan ($20/month), unlimited texting ($15/month), and I'd *STILL* come out on top vs. a 600-minute AT&T plan.
2. Some people may be in an area where the majority of the people they call are on the T-Mobile network, and as such they use less minutes if their T-Mobile calls are baked in.
3. Still others have an axe to grind against AT&T, don't like their customer service, or what have you, and are avoiding AT&T on principle.
4. coupled with #3, it's probably the simplest reason: "Because I Can".
5. T-Mobile might not be able to have an exclusive contract with AT&T, but every time I've called them with a question about using it, they answer "We don't officially support this, but here are all the steps you need in order to do what you want to do."
6. AT&T won't let you tether your iPhone to your laptop to use it as a modem. While you're stuck at EDGE speeds on T-Mobile, they really don't care what you do with your data plan, so you can tether away without getting any love letters from people threatening to cancel your service.
7. If you're an AT&T customer and you jailbreak for things like MMS and tethering (see above), your warranty is already voided and they won't help you anymore. So again, if they wouldn't help me anyway, I'd unlock it on principle just to put the screws to them. Yes I realize that if I were an AT&T customer they'd still have my money, but if it's an opportunity to stick it to them and it wouldn't cost me anything, it's well worth the time investment.
Those are just a few.
Joey