They've still not done this with phones, so they've left a giant hole for Android to fill, which in turn makes the platform more attractive to developers.
For example, the shuffle seemed more aimed at people who wanted something easy and light to clip on while exercising.
The nano seemed to cross-over slightly with that market, and also attract people who wanted something tiny but full-featured. (I.e., it was the size that differentiated it, not price.)
I don't think many people thought "Oh, I'll get a shuffle because I can't afford a regular iPod."
The fact that Apple continues to offer the previous years' iPhones is actually kind of unusual with regards to the structure of their past product lines. It's how they are trying to cater to different price brackets rather than use-cases.
As for use cases, phones are all for adults, or rather every age group. Apple and others have not brought out products that are for kids or old age folk, as in specifically for them, e.g. with restrictions on in app purchases and porn where some 'mothership' device can be asked to yay or nay an action. Also, there 'should' be a phone for old folks that is full featured yet only presents a simple dialler on the home screen.
Speaking for myself, the shuffle is definitely aimed at my use case (I actually used a zen stone). I want to control my in-car music by hooking up a player with a purely tactile interface. Using my phone means every time I want to interfere with the player, I have to look at the screen... a huge step backwards in usability.
The zen stone had an adavantage that an ipod shuffle might or might not also have... no one ever broke into my car to steal it (although it was taken when they broke into my car to steal my backpack).