You have to ask yourself a number of questions:
What sort of band are you in, and what sort of gigs you do.
Are you playing 30-45 minute sets down at the local? Or are you doing 2-3 hour stints at functions (weddings etc.)?
If you're doing the first option, then no. You don't need to buy and take a spare bass. Make sure your main instrument is looked after, strings, batteries, plectra, etc. and you'll be fine (always carry spare strings & batteries if you need them though).
If you're playing in a function band and doing a couple of long sets per show, then yes, you do need a spare instrument, as it's a completely separate premise. You're being paid decent sums of money to provide entertainment, so you need to be professional and be prepared for any eventuality. And if that eventuality occurs, then the switch should be as quick as possible.
I would assume that the first option is the case, and you don't NEED to take another bass to rehearsals & gigs.