most people fail to realize that glass isn't that hard
sure diamonds will scratch glass, but so will certain types of metal, such as sand from the beach, certain kinds of concrete, etc
Here's the best kind of advice i can give you
a) if you are a mindful person, and remember to not carry your keys and your iPhone in the same pocket then you don't need a case
b) If you are a person that tends to drop things, is concerned about scratching their phone, then get a case
I use to have a shield on my phone but now its naked.. I really like the feel of the phone without any case at all.