As an amateur, you don't really need a backup camera, although you may want a second or backup camera. A professional has contracted to provide a service, and is legally responsible for mitigating all of the factors that may prevent him or her from providing that service. A wedding photog, for instance, can't simply tell the couple on their wedding day that they're going to have to reschedule because his shutter's stuck.

The point of same-brand (or same-type) backups is to carry as little as possible, and to ensure that as little thinking as possible needs to be done to adjust to the backup if you need to use it. Someone shooting, say, Nikon D4s as their main bodies may carry a D3/D3s or a D700 as a backup -- it'll be familiar, have the same field of view with the same lenses, and so on. A D7000 shooter might carry a D90. A Canon 1Dx shooter might carry a 5D. With things like flashes, it's more normal to carry the same model so that transferring settings is a no-brainer.

One doesn't usually carry duplicate lenses (except in highly specialised types of photography); having lenses that can, in a pinch, cover one another is more normal (that is, a mixture of primes and zooms covering the same focal length range). It's only when you have a signature "look" that depends on a particular lens that you'd carry an actual duplicate (usually) -- for instance, Ryan Brenizer without his 85mm/1.4 would still be a great photographer, but his customers wouldn't get the Brenizer look.

If you just want a camera for when you can't use the one you have now, and don't have to worry about getting immediately into the flow, then there's no harm in using a different type of camera. You may, in fact, want something different; something that's easier to have with you all of the time than your DSLR kit. (You may even want to have a couple of options, one of which may be a decent camera phone or compact camera.) Of course it's going to affect your post-processing procedures, but it's not like being dropped by parachute into a country whose language you don't understand and with one leg amputated -- you can adjust. It's only when your living depends on getting things right quickly that you really need to worry about it.