I bought a 5D1 in 2007, and moved my 20D to back-up body status. Then I bought a 5D3 in 12/2012, moved my 5D1 to back-up, and the 20D collects dust.
Using the 5D1/20D combo was easy, because their controls and menu systems are virtually identical, and they use the same batteries. Also, the 20D's sensor is higher resolution and an APS-C crop of the 5D1, so when I used both bodies together (to avoid lens changes), I would mount a wide-angle or standard lens on the 5D1, and a telephoto on the 20D.
The 5D3 is a different beast, entirely. When switching between the two 5D generations, I'm constantly having to remind myself which one I'm using, and adjust my interaction with it accordingly.
There isn't anything wrong with your plan to buy a 5D for use as a 2nd body, but I wouldn't do it. For the money, I'd rather buy a more modern crop body with controls and interface more similar to the 5D3.
Personally, I'm (patiently) waiting for the price of the 5D3 to drop to about $2,500, and then I'll buy a second one. Sadly, it seems like the price has gone UP lately, what with Canon's recent MAP policy enforcement and all.