Since I have kept a backup SLR camera body, and probably how most photographers do it, is when you upgrade camera bodies, your old one becomes your backup, since you know it works well and know it’s history. When I bought my refurbished Canon 1d Mark II in 2005, I kept my 10D as my backup….there was nothing wrong with the 10D but I fell in love with the 1D focusing system! That situation was not ideal, but it served the purpose. The interfaces and overall operation of the 2 different cameras are nothing alike, completely different! Therefore, in case of failure of the primary camera, you have to adapt quickly, if it happens in the middle of a shoot…..which it will, every time LOL! I had my Mark II for about 2 years and the shutter curtain band broke while I was doing some model headshots, so I was glad to have the trusty 10D, which I used until the 1D came back from Canon’s repair facility. If I didn’t have that backup, that shoot, and other shoots would have been off for about 10 days.
There are different schools of thought on backup cameras…..some have one camera body that is strong in one area, and the other has another advantage they like…..for example, some shutters are quieter than other, for Catholic weddings, I can tell you from experience that makes a difference, as far as church officiants staying off your back. So, some photographers may find it handy to go back and forth depending on the occasion. I go to networking events sometimes talking with other Nashville wedding photographers, some who have studios as well, and I have heard both sides of it. It’s my personal preference to easily be able to go back and forth, meaning interfaces of my primary and backup bodies are similar enough to easily adapt when necessary. I bought my 1D Mark III this past Fall, and use it as my primary body…..my Mark II serves as my backup. The interface on the Mark III is a little less cumbersome, but overall not worlds different, so I can easily adapt. If shooting a wedding, it all moves so fast, there is no time for getting familiar…..you just have to DO, lol. You don’t have the breathing room for weddings or commercial shoots as you would for a portrait session. This situation feels ideal to me now…..either body is very capable, and neither take much getting used to for me.