The most stressful shooting experiences most us will have will be in competition, and there, familiarity with one's guns really pays off.
For semiauto pistols, having a good grip and recoil control is a big deal, and a lot of it depends on nuances such as where the safety and other controls are mounted, the thickness of the gun's arsepart and trigger morphology. Getting mixed up between guns that may be superficially similar costs you a lot in competition. To wit, last night I trained on some stages with my SP01 Shadow and SP01 Phantom, and despite being the same size and shape these two shooters require completely different grip methods, despite being compatible vis-a-vis rigs, mags, ammo, bells and whistles.
For rifles, I don't have much experience, but I was brought up on M16s and Galils, and had no major problem with either platform in killing paper targets. The first time I held and shot an AK47 I felt intimately familiar with it immediately. These experience relate to basically stock guns without any aiming devices hanging off them - familiarity with these is certainly something that does not want to learn for the first time in a gunfight.
So yeah, if you are carrying a gun for an unimportant reason such as protecting your life and that of others dear to you, or for a crucial reason such as competing, it is good to be familiar with its nuances and not get mixed up when split seconds count.
Rust and bureaucrats. Freedom and vigilance. Front sight and trigger. Kindness and firepower. Situational awareness and tolerance. Safety and concealment. Taxes and allegiance. Love of man and surgical marksmanship. Once a soldier, always a soldier.