Why wouldn't you use a cropped sensor?
Think of it this way. Let's say you're shooting a subject with an 80mm lens on MF system with a 1.1 crop. You're ten feet away and you get a nice upper body shot.
If your MF system goes down and you now have to pull out your 5D, you'll probably have to stand about 15 feet away with your 85mm to get a similar shot.
On the other hand, if you have a 40D, you'll have to stand about 20 feet away to get the same shot with an 85mm. (I made these distances up. Perhaps someone can jump in with the actual differences.)
Of course, as you get farther away, your perspective changes which changes the look of your shot. Your other option is to shoot with a wider lens, but that also changes the look of shot. Since the 5D sensor size is closest to MFDB, you'll have the least amount of change if you need to switch from one to the other. Since you're using the small format as a backup, I'd think you'd want to minimize the difference between formats. Hence the 5D.