I have multiple "back-ups";> )-- then there's my "action" camera (7D), my ependable "kayak" camera 50D w/18-200, my light weight walk around outfit 60D (yet to acquire) etc. Also needed are my "car" camera-- need a G12, my lightweight hiking outfit-- m43 G1 w/ 14-140. And I can go crazy with this for multiple lenses and back-ups;> )
Seriously, I can certainly see the need for back-ups when doing pro work e.g weddings where you only get one chance, but other event work as well. I just covered a yacht regatta and had but one chance to get it "right", so 7D w/18-200 and yes, I used the whole lens range. Shooting was fast and furious at times. Back-up 50D w/ 15-85.
...and so it goes. cheers. ernie.