I'm not gigging at the moment, but IMHO whether one would need a 2nd as a backup should be viewed as follows:
1. You can comfortably afford it. This really is the #1 factor, even if you don't really need it and will never use it. If you are fortunate enough to be able to easily afford a second as a backup, just do it. Cliff and crew will appreciate your business.
2. You actually need it. If there is something complex in the way your patches are set up or specific effects unique to the Fractal family that you really need to achieve your performance goals. For example, if you heavily rely on the pitch effects and multi-delay by the time you build up a modular backup system to recreate those effects you'd probably be around the same price as an AX8. However, if you're doing pretty standard rock stuff, you could probably get by with a Tech 21 Fly Rig in a pinch at a huge savings in terms of purchase price, size when transporting, and safety concerns (less risk leaving it in the car/van or offstage).
