Its rarely practical, only if you are a pro or on an extended mission. Mostly its ego driven. Some gear fanatics love to gain what they imagine is status, and of course two cameras have to be better than one don't they? Its comical when you see amateur Canon and Nikon guys with two around their necks, with Leica its just tragic.
Steve
Steve - I respectfully have to disagree and state: this is nonsense.
When I used (and still do) SLRs, lets say at a racing track, I have one body with a tele (70-200 or a longer prime) and another body with a normal or wide lens.
It is simply impractical and uncomfortable to switch lenses around.
The same fits to my use of RF cameras. I have a small daily bag, that fits a 15" laptop and two Leica cameras with lenses attached for work.
I choose the two cameras and lenses, to complement each other well - one with a wide, the other with a normal lens.
One camera, which I prefer to shoot (M8.2) and another one, that fairs much better in low light (R-D1 or a film Leica).
I don't know, how you base such a reasonable use of more than one camera on your "ego - statement".
Just because, one does not earn the major income with photography and uses more than one camera qualifies for having a bad character, does it?
And for the record - my cameras are taped and I don't mind the scars, as I prefer, to not get involved in camera talk, while shooting - quite contrary to the ego driven usage of "luxury gear" - isn't it?