I would vote other, since it depends on the situation.
I shoot corporate and commercial work and have full backup for bodies, lenses, strobes, light meters, computers, and other essential gear. Since the cost of equipment, location, assistants, models, catering, studio, etc, don't go away if something breaks, and it can easily run into a couple grand a day, I need to be prepared to keep going no matter what.
The same holds true for events that can't be recreated, or would be very costly to do so. A portrait can always be rescheduled very easily, and at worse you might have to give a refund, or have a disgruntled client. But at least you won't have to refund the fee, have a disgrunted client, and shell out $5,834.32 in expenses because you couldn't deliver.
I even backup people
If a shoot really needs one assistant to function, I book two. One experienced first assistant, and another second assistant. If the first assistant gets hurt, sick, car breaks down, etc, I can usually function with a bit more workload on me, and the slack taken up with the second assistant. If it needs two to function, I book two experienced assistants, and a fresh assistant or intern. Same with makeup. I don't double book talent, but the agency is usually ready to send over an alternate should something happen. So they have backup in a way as well.
The more serious or costly the shoot, the more important the backup equipment and people are.