One doesn't need a backup for every piece of equipment. One needs a solution for most of the possible problems that could be encountered. If you have multiple amps in a rack, you can potentially rerig your rack to accomodate an amplifier problem, for example. Also, one needs to consider the odds of failure...you wouldn't need to carry 16 extra lights if your event is using 16 lights...the odds of all failing at once are pretty slim. You would be wise, however, to carry an extra fixture, an extra lamp or two, perhaps extra socket or other integral component. It would also be smart to have some simple tools for making a quick, simple, onsite repair such as a broken plug end or wire inadvertantly pinched in a door and damaged...wire stripper, electrical tape, etc.