I am not so super strict about firing with only one gun - I usually start out with my Ruger .22
But for me, I have limited resources, I only have so much time I can spend at the range, I also have to pay from range time, and the money I spend on ammo is also a factor.
I want to make sure I can use my primary handgun instinctively and without a lot of thinking and I only have so much time and money to accomplish that.
If I were rotating through a bunch of different firearms - some with manual safeties that flipped down, some with manual safeties that flipped up - some with levers that are decockers, some with no levers at all some with magazine release levers some with magazine release buttons... I think I would end up never mastering any single one of them to the point where when I needed to use it instinctively I could. If there are people out there that feel confident that they can well good for them. Some people don't even have a "primary" handgun. They've got a rotation of night stand guns, a summer gun, a winter gun, different guns they take with them in their car... and they're all different makes and models.
I want to devote enough time and effort to make sure I can use my primary handgun. On the other hand, I enjoy shooting. So I like the experience of shooting different pistols. Occasionally I will give myself a "fun day" at the range where I do nothing but shoot whatever I want. But usually I focus on making sure that I can fire my primary handgun accurately.