I pretty much echo the sentiments of the majority. I always take two basses, generally my four string Shuker jazz and a various 5 string (soon to be 5 string Shuker jazz!). It's for two reasons - some of the tunes my band play require a five string, but I generally prefer playing a four, so just use a five on those particular tracks. Also if anything went wrong, it's far easier to switch to the other bass and continue with a minimum of fuss. Plus, really, after amps/cabs/flightcases/bags etc, what's one more bass in the car?
Also, as all the gigs I do are paid, I think I have a duty to be professional and have all "bases" covered, so to speak.
Interesting stories on here, especially the two about broken E strings on doubles, what a nightmare!! I don't suppose many double players bring a spare to a gig!
Regards
Mike