I never owned a case with any of my iPads just smart covers and never encountered any scratching. I don't use cases on my phones either so maybe I'm the wrong person to ask
I use my mini w/o a case, just a screen protector. Previous one (bought Day One last year) looked brand new when I packed it up yesterday. It's pretty durable as long as you are not a slob and toss it around like a football. But if you travel I would get a carrying case for it. You can get the great Case Logic LNEO10 for $14. Amazon sells it I know.
I did this on my ipad 2. Did the trick since I knew i was protected from day-to-day scuffs and the like, thing looks brand new still and I never even noticed the skin.
Tried a smart cover then a smart case on my iPad 2 and eventually settled on a cheap soft TPU back cover.
However, my new Air feels so light and 'fragile' I've gone for another smart case. Made it feel almost as heavy as my old 2 but for now I'll keep it. Guaranteed in a couple of weeks the newness and must protect it feeling will wear off and a back cover will be bought!
You could always buy a back "screen" protector.
Prevents scratches and you wont have a case.
I bought one from xtremeguard.com
3 dollars for a back only protector if you use the discountcode: 85off2
Getting it on is bit crappy and after a while the edges get dirty and a bit loose but for 3 dollars I find it ok.