Is it necessary? No, it's not.
HOWEVER, by not having a case, and screen protector, you are leaving yourself at a much larger risk to get scratches, nicks, smudges, fingerprints, dirt, and dust on your iphone. Then, if you were to ever drop it, not that you would, but what if you did, imagine.....without a screen protector the screen could be all cracked or scratched up. With it, maybe the screen protector helps take some of the damage? Same goes with a case....
If you think you'll be fine without it, then don't get one.....
Personally, I know i'm not always as careful as I could be.... particularly when i'm out at the bar or something. So, I want all the protection I can get.
Do us a favor though, if you decide not to put one on, can you post on here in another 2-3 months and let us know how many scratches are on the screen?

j/k good luck man.