We sat down to work out if we could get rid of one of our cars because it costs so much to run them. I walk to work and DD gets the bus to school, but she can't get to most of her activities on the bus.
DH can get home from work but not there during the week, and get there but not home at the weekends, so that doesn't work, and currently DS is working with him, so the car is cheaper than 2 lots of train/bus fare. If he took our only car to work he doesn't get home in time for us to go out.
The other issue we used to have when we shared 1 car years ago is that he likes to ride around for ages with the petrol light on (despite running out of petrol twice abroad in really scary situations). I get twitchy once the needle hits the red bit. So I would fill up the car, he would drive it for a week, I'd get in it to go somewhere and the fuel light would be on.
If he does fuel it he'll only put in £20, or 20 litres, while I'd fill it right up. So I was always the one filling it up, and always the one caught out if we needed to go somewhere early.
I also used to leave 50p in the front for parking (this was a long time ago!). He would "borrow" it, and not say anything, so I'd get to the shop and find I'd got no money to park. I also have stuff in my car like coats, spare jumpers etc while he likes to clean his out completely. So I'd make sure the car had my emergency supplies, he'd come along and empty it, and I'd only realise when I needed a change for a wet child to be faced with an empty boot.