I've never done that because it doesn't make sense to me. One would buy an iPhone to combine both devices, why would you want to go back to having two devices?
There's plenty of reasons:
1) Pricing: I can get a lot of smartphones for the same $200 as an iPhone, but the dataplans/text plans for the iPhone are a lot more expensive than other options. Example: I can pay $60 a month with Sprint for the same service that would cost me $90+ on ATT. That pays for an iPod touch real quick, with plenty left over afet two years.
2) Business users: the iPhone is a fantastic piece of hardware, but it lacks some of the business integration of other devices (though the Exchange integration back in 2.0 solved some of that).
3) ATT: Some people don't like certain companies for past issues (I for one hate ATT and won't give them my business).
4) Keyboard: the touch screen on the iPhone is great for browsing or games but, personally, I think it sucks for typing. I like a real keyboard for email and text.
5) Battery life: I rely on my phone for work (email and calendar in particular) and I wouldn't want to run it down while watching a movie or playing Flight Control or whatever. If I kill my touch battery doing those things it's not a big deal, though.
Those are just five major things I could think of right off hand. Personally, I'm very happy with my new Treo Pro and iPod touch combo, especially since I can use my Treo to create a wifi network the touch can access to get online for games, app downloads, etc. (I only do this when I have access to charging or a charged spare battery handy, as mentioned above I don't kill my phone for "play"). Plus my monthly bill is A THIRD what I would pay for an iPhone. I "paid off" the touch in six moths or so (bought mine back when they were brand new and around $400).