The pros and cons of a backup camera

I do think a 2nd camera is useful, but indeed Nando, then you have to use it too. For my current R7 I had 2 bodies, the 70D and 80D from Canon. On one body I put a wide-angle zoom and on the other a telephoto zoom. At every location I went to, I used both cameras. Changing lenses was never necessary, which meant I was also less at risk of getting dust and sand on the sensor.
I traded in the 70D and 80D to be able to buy the R7 with the standard kit lens.
So far I have no regrets about that. Even though I now have to make do with 1 body. The R7 has brought me a lot of advantages. Moreover, I am physically no longer able to go out with 2 cameras at the same time. I now go out with a particular goal sooner and choose the lens that best suits it to take with me.

Always had a 2nd camera, yes if one stops working you have a backup, but I use them simultaneously and separately. depends on the situation. it's nice not to have to keep changing lenses and my 2nd is a crop camera, a good combination with full frame. my 24-70mm then becomes a 38-112mm, ideal as a portrait zoom.

I initially had a Canon 6d markii. Afterwards I also bought an r6 markii. I mainly use the R6. I now use the d6 as a 2nd camera. When photographing first communions or weddings. It is sometimes quite handy and convenient.

During my vacation, the flex cable of my Canon lens broke. I hadn't brought another lens, but fortunately I did have a compact camera, so I was still able to take photos.
Especially when doing photography on assignment, a second camera is useful. Just imagine suddenly seeing the message "error" appear during a wedding.

2 cameras ?, this weekend to Oostduinkerke (shrimp fishermen's weekend) one camera R5 (with 24-105) the other 500D (Lens 100-400) also bringing a tilt/shift, with tripod(monopod)
I do have to say that I have a kind of caddy for this, which I can also use at the beach.
Loaded indeed, am I going to use everything?
Going to test this combination for later URK and PARIS.
Indeed I have found that I only use 1 camera.
My bag is handy though, so both cameras can be switched quickly. The caddy can also be used as a little seat.
So, an experiment.

always at least 2 cameras with me. But for different lenses, one for the 18-400, one for macro or wide-angle, depending on what I'm going to do, and sometimes a third for an analog lens. Handy if one breaks down, but that's not the goal ??

When I travel, I always take 2 cameraswith me. My R7 with a telephoto lens and my 90D with a wide-angle lens. That way I don't have to change lenses.

Yes, if you use your second camera at the same time it has a use, but not as a backup camera? Strange way of reasoning. I have always brought my older camera as a backup on assignments. Used it as a second camera very occasionally, but that was really an exception. It has benefited me though: twice a camera broke down during work. Once during a wedding and once during a portrait session with a state secretary. If I hadn't had a backup I wouldn't really have come across as a professional (apart from the quality of my work). As a pro you should equip yourself accordingly!

I also have two cameras: a Nikon D810 (full frame) and a Nikon D500 (crop).
The full-frame is mainly for portrait and event photography, and the crop obviously for nature photography. Only one of them is ever in use. A matter of planning properly for the type of photography you're going to do. My 'arsenal' of lenses never comes along either: only the ones I need for the planned activity. Only very occasionally do I regret that, when I miss having one alongside the one I brought, but I'd rather have that than lugging around a dozen kilos every time. A matter of mindset and not being susceptible to FOMO

But it was about a backup camera.... no, after all these years I still haven't had a good argument for one.