Why would anyone need two DSLRs?

My wife is puzzled by me wanting to hold on to my Canon 40D when I get the Canon 5DIII.  She thinks getting the ~$200 or whatever it would be for the 40D would be worth it...

1) A backup camera is always useful.  Cameras break or have issues just when you need them. 

2) Two cameras with two lenses are useful, saves the lens switch.

3) One is crop, one is full frame, probably not that I'd prefer a five year old crop to a modern full frame.

4) The 40D is going to be smaller and more portable than the 5DIII.  This is probably the biggest reason to hang onto it.  The 40D is 740 g, the 5DII is 810 g, the 5DIII may be larger.  Hm, this size difference may be smaller than I thought, the 1D and 1Ds cameras are much bigger (alsmost 1200 g), the 5DII no so much...