I bring two basses, two cabs, two heads, extra cables and strings, duct tape and tools to every gig. I've also got picks, guitar strings, drumsticks and a DI in my gear bag in case one of my bandmates has a problem. I may be paranoid, but I'm a firm believer in Murphy's Law.
I would discourage covering your band-mates backs gear wise, why? because they will slowly begin to rely on you completely for their gear back-ups etc, It's much much much better if each individual band member worries about his own gear. Trust me on this one.
