Nope. Smartphones today typically do NOT need a case. Well, unless you have an i product from that company with a fruit as its name. :-) Those glass sides seem to break quite easily.
That's not the case (pun intended) with most new phones however.
My HTC Sensation 4G doesn't need a case with it's Corning Gorrilla Glass screen and the design of the body. The glass is curved at the edges and the body of the phone is completely removable. The glass does not scratch or break.
I've actually demonstrated this by taking a fork and a knife and jabbing it all over the screen. Not a scratch. I've demonstrated how strong it is at a bar by throwing on the floor multiple times. Yes, I had had a few. LOL.
Even so, the most damage it's seen, and believe me, I drop it all the time, is some scratches on the corners of body. If I really want it to be just like new I could easily replace the body as it contains no electronics at all and it would be just like new phone.
So, there ya have it.