it's fine to have a 5D2 as a backup, it's just that essentially you have a similar camera as a backup. The 5D3 was a much-needed evolutionary change from the 5D2. If you want something more versatile, a 7D would really complement it. As far as a backup camera to a 5D3, the 5D2 is more appropriate.
I wonder what Nikon was thinking with their D800... very good camera, but it seems very different from the D700.
