To case or not to case, that is the question.
Whether 'tis nobler in the mind to suffer casless
The slings and arrows of outrageous fortune
Or to take arms in a Lifeproof against a sea of troubles,
I have three cases:
Lifeproof - $80 retail
Apple shell case - $40 retail
Otterbox Commuter - $40 retail
Yes, I bought all of them within a week of purchase of the 5S. I only use the shell case from Apple and even then it won't protect against everything that could happen to the phone. The case, if anything, protects from mild surface scratches that could happen to the phone when not in use.
To each his own.
Edit:
Longevity versus Condition
If your plan is to keep your iPhone for 12 months to 2 years and either trade-in or sell the device to offset the purchase of a new iPhone, a case will preserve its physical condition. What it can't or won't do, however, it assure you that the device will last any longer than a phone without a case.
Take a phone in a standard case (sans water/dust/scratch/impact) resistance you can find in a common retailer against a phone without a case and submerge them both in water for 30 minutes. Take them out, "rice" them for 24-48 hours and attempt to use them. Depending on your "luck" and where water was able to enter the submerged device after 30 minutes, both devices probably won't work / work at 100%.
Just because an iPhone appears to be in better condition does not mean that it is in better condition. This is why you cannot judge a book by its cover.