I was going to Arizona from England so space was a premium, especially as I had no intention of letting go of my camera gear so it had to meet the overhead dimensions. The other half was taking her toys so declined to carry any of mine!
I had a nearly new 40D and my 300D a few years old.
I rationalised my kit bag, removed the 300, put it back, took something else out, put that back took the body out 3 times until I finally settled on taking the 300D.
Part way through our 14 day stay in AZ we were at the Out of Africa Park, been in the park 15 minutes and bam err99.
Remove battery reset and thank goodness the picture is there, click err99, oh well back to the car for spare body no probs except the frame rate and buffer meant I missed more of Tiger Splash than I would with the 40D, however I got some pics that I wouldn't have without a spare body!
So yes having a backup is a good plan, however you may be able to sell the 7D to raise some capital and spend 1/3 of that on a lesser body that is good enough to sit around waiting for maybe never getting used, however always take it with you.
The err99 on the 40D was the mirror and shutter, or that is both the bits they changed.