I think it is important.
Maintaining your instrument is your part of the equation. You keep your strings fresh, you keep batteries replaced, you bring spares of those. But what if someone steals your bass before a show? Or maybe it's on a stand waiting for you and someone knocks it over and breaks it. Or maybe some well-meaning person tries to move it out of the way for someone moving cabinets and drops it. Or maybe some stage-hand drops a speaker cabinet on it. Yeah, you beat them severely, but if you don't have a spare, you are going to have to try to borrow one from someone else, or run to a music store, or just not play.
You never know. Good to keep a spare in the car/truck/trailer/van.