In a different situation with some intersection to your questions...years ago my agency allowed us to carry any firearm, owned or issued, over .380 caliber and with which we qualified quarterly. This applied to firearms carried on and off the job. So you had agents carrying single action cocked and locked, double action revolvers and a wide variety of semi-autos from Sig to Beretta to Glock and on and on.
Eventually the agency decided to mandate that the issued firearms would be the Glock 19 and 26. The reason being so that everyone was familiar with the same weapon platforms. Imagine being in a situation where you had to pick up a partner's weapon and bring it to bear only to find that you didn't know where the safety was or how to operate the decocking lever.
There is a certain amount of truth in the old adage about fearing the man with one gun but the bottom line is familiarity, comfort and competence.
I enjoy firing different weapons owned by friends and neighbors but when I drill I drill with the firearms I intend to defend my life and home with if necessary.
I consider the Glocks (19 and 26) that I've been carrying and training with since the late 80's to essentially be equivalent to training with one gun.