I kept my 4s in an otterbox commuter case cause I knew it was going to my wife when the 5 came out (she wanted it rather than the "larger" 5).
Now that I have the 5, I have an applecare+ warranty on it so it'd cost me $50 to get a replacment. There are decent cases out there that are almost that much money!
I recently ditched the Incipio Dual-pro case that I got for it for daily use (love that case by the way) and got a Best Skins Ever skin. I love the skin but it's peeling a bit on the bottom by the headphone jack. I have yet to contact them about it but from what another user posted, they'll take care of it.
Anyway, the way I see it is that when the next iPhone comes out, I'll either get that and give this one to my wife or she can have the new one (which she never cares about) and I'll keep this one. Either way, a case is a bit overkill for me. The phone is SO THIN without a case and fits in my pocket awesome.
Others may not have an applecare+ warranty or opportunity for an upgrade every year and a case may be necessary.
It's a tradeoff and no two situations are similar. People need to assess what works best for them and do it!
Have you looked at all in the Accessories subforum? There are countless threads on various cases. The "why" is for you to determine. What do you need/want? What's your budget?