How many times have you had 1 series cameras fail? Since 2004 I have had one, and that was because I dropped it. This talk of "I need a backup" is largely bullish!t.Maybe I can explain this to you:
After I have paid over $20 k for a 600mm IS II, 400mm DO II and a 100-400, I cant afford to buy 2 x 1DX MKII. I need to have two bodies, because one of them is a backup if one failed. So there went another $4k on bodies plus of course grips and batteries so say 5k in all.
Maybe in the best of scenarios I would have wanted one of them to be a 1DX MKII but, I definitely wanted one of them to be a crop body. When you are shooting 1000-3000+ images a day and you crop most of them anyway a FF is quite pointless, you can seemingly never have to much reach.