batteries on the iphone deplete astonishingly quick; it's pretty much a choice between a phone or an ipod. When you're catching the train, driving for a long time, walking/running, public transport or just chilling, you can't use an 'iphone' for music. I have an iphone and an ipod (not an itouch) and i never listen to music on the ihpone cuz it dies wayyy to quickly
I use my iPhone to play music and it rarely/never gets below 75% battery.
the ipod app on the iphone sucks. an actual ipod is much better for use as an ipod. an iphone is good for everything else. also like mattyrussell said, the battery drains way too fast. and like anthony said, it's much easier to control an ipod when you can just feel for the buttons.
Having both is the modern tech equivellent to a Mullet. Business and party with all round douche holding it together.
And an iPad ;)