I just got the jet black 7 and have been going case-less so far. I didn't use a case on the 6 it replaces, but I am concerned about the micro abrasions. The way I see it though there's no way to really avoid them. If I put a case on dirt between the case and phone will cause them. If I don't use a case anything can cause them. I just have to accept that this phone will soon be scratched up.
I'm more concerned with the dropping aspect; the day before moving to the 7 I dropped my 6 corner first on the street after taking a picture. It wasn't as damaged as I anticipated it would be, but if that were the jet black I assume it would look much worse. I do not look forward to that day.
Anyway, I've only had my 7 a few days and so far so good. The 6 was case-less for 2 years and was in ok condition. A small crack in the corner of the screen, scuffs and dings around the body, but being able to hold the phone and not a brick with the phone inside is much preferred to me.