Originally Posted by gwmac
I hear what you are saying but offering plastic is more than just about cost. It also clearly differentiates the lines. I disagree with the decreased durability and sturdiness of plastic though. I think the the 3GS is probably more sturdy and durable than the 4. I also fail to see how using plastic necessarily reduce space inside the phone either. It might make it slightly more bulky on the outside but people expect compromises when paying significantly lower prices. But I think the main reason to avoid aluminium is clear product separation although multi-colored aluminium or other metal would also offer that as well.
You're probably right about plastic differentiating the lines, though I'd have to point out that Apple, at least so far hasn't really had multiple lines of products, they seem to mainly have the one line - premium. The only thing resembling multiple lines is previous generation products, but I don't recall the last time two products were introduced with one marketed as a premium line over the other. But who knows, things may change in the future.
As for the sturdiness of the 3GS, sturdier than the 4 maybe, not the 5. And that's really mainly because the back glass was prone to cracking but I think the assembly as a whole was sturdier.
Like I said, plastic will either make the phone bulkier and/or reduce the internal space if attempting to use the same external size as an aluminum case. There's no way around this, plastic can't be made with the thickness and tolerances that aluminum can, and if given a choice what would you rather have, more plastic or more battery? I suppose they could offer less battery life in a cheaper iPhone line with plastic case, but it hardly seems worth the trouble just to differentiate the lines. I think I like the other idea mentioned in this thread better which was to use a case similar to an iPod touch - it avoids the bulkiness and it also clearly differentiates the lines. It's just that I see this plastic iPhone idea mentioned so often, and no matter how you spin it, it doesn't make much sense to me.