Well I would like to have a backup camera body, and I currently shoot with both the 5D Mark III and 1D Mark IV. There is someone that has offered me his 1Ds Mark III that was his backup body (he pre-ordered the 1D X) for $4000. It wasn't used much because his backup wasn't used much, maybe has 5-7k actuations. He did send it to Canon to have it refurbished, which it is. Well, I'm not sure it is worth it. I can get a 5D Mark II for 2k that I wouldn't use as much as my current cameras, but I've never even used a 1Ds series camera. Would it be woth it? What can the 1Ds III do that say, the 5D Mark III and II can't? Thanks for any input.

P.S. I can afford it but the issue is whether it is worth it or necessary. Ah maybe I should just sell my 1D Mark IV and get a 1D X