From personal experience, the iPhone 5 is very susceptible to moisture damage. You don’t have dunk it to ruin it. Leaving it in a wet backpack can do it in.
I’m not suggesting everyone needs an Otterbox but be aware of the conditions you are exposing it to and protect it accordingly.
I love the feel of an unprotected iPhone 5 but I think I will get something to give the case basic protection.
I got a bumper case for my iPhone 4S and carried it in that for the first 8 months or so, until the rubber started to split from wear. I use my iPhone a lot and constantly taking it out seems to have worn it more than other friends who had the same case. The bumper was a very minimal change to the form, which is why I got it. It did have more friction, making it harder to take the phone out and put it back in my pocket. I decided not to use another case after I ditched the bumper and have carried my iPhone “naked” since then.
It’s made basically no difference. I use a bit more care in putting it down, making sure not to put it directly on our glass table and things like that. Then again, I don’t tend to drop my phone. If you’re a klutz, maybe you should get a case. But if you pay attention and are careful in handling your phone, you probably don’t need one.
Reported