The batteries you buy off the shelf have a charge. They tend to hold their charge for quite a while without deteriorating. However, keeping a spare in the car means carrying extra weight, which means spending fuel just hauling it around for "Just In Case".
IF you carry a spare it will need to be bolted down so it doesn't topple over and spill acid all over everything. AND you need to keep it bolted down so that in the event of an accident it doesn't become a projectile and injure a passenger.
Yeah, it's OK to keep a spare. I used to keep a spare under the hood, bolted down and wired so that it would constantly be charged but would only be used when I pressed a button. It came in handy a few times but the added expense of the custom wiring for the few times I left my lights on didn't versus the handiness of it being there - well, it's not going to be a big expense, but honestly, it's not really necessary.