I do have multiples and spares of the same camera bodies, the ones I use the most (Nikon FE, Minolta Autocord, and Holga). In the first two cases, I got the second or third (or more) bodies because it was cheaper and faster than getting a repair done to the camera that was broken. Occasionally though, I would send out the camera for repair, only to be told that they no longer have the parts needed. As a result of that, I've just kept the cameras that are no longer working for whatever reason so that I may use them for parts, if and when needed, in the future. But I also have a working backup for each kind of camera. As for the Holga, well, I know it's cheap, but because of that they are all different, and once the plant shut down I got some extras just in case that was really it for the brand (it wasn't, but I didn't know that then).
However, when it comes to shooting, I often travel with 2 or 3 cameras (usually the ones listed above - I might switch out the Autocord for a Rolleiflex, but still, a TLR). The cameras allow me to do different things (different film stocks/speeds and subjects) and also act as a backup in case something fails - which has happened on occasion. If I'm just shooting locally then one camera is fine, but like others have said, I would never travel somewhere with only one camera. Even if my primary camera fails, I know that I have others with me that can take over that role if needed. Some other poster mentioned the fact that it's hard to find camera stores where one could replace a lost or broken camera - I had that experience in Paris - my Nikon FE was out of the running, but I needed 35mm to shoot infrared. So I spent almost an entire day going to various camera shops in the city to find any (preferably) mechanical Nikon camera that I could use. I only found a couple, and they were sitting under two inches of dust in a very sunny window, so I took a pass. Here in Japan that wouldn't be problem though!