There are two major reasons not to store a gun in a holster.
1. The leather attracts and holds moisture from the air and in conjunction with the chemicals it was tanned with can rust the metal.
Also, the leather will wear the guns finish more.
2. Leaving a gun in a quality, fitted holster will stretch the leather causing the gun to no longer fit properly. Since the gun will be able to move around in the holster more, the finish will wear faster.