Placing a case on a phone is like putting a plastic cover over your couch. I don't get it personally. Use and enjoy the disposable hardware and then get a new one a year or two later.
Time to failure is an exponential distribution, whereas number of failures in a given time frame is a Poisson distribution, if and only if the hazard rate is constant, which it typically is not, due to wearing of certain components. It's usually a bathtub curve.
(your friendly ex-statistics lecture adding some precision)