All depends on where you live and what you drive, imo.
I normally keep a saw locked in the toolbox of my truck. I can think of at least 3 times in the past year that I've been damn happy I decided to keep a ready-to-go saw in my toolbox.
In fact, just earlier this week I drove over to a coworker's house to borrow a tool. I start up the driveway and realize that a tree had fallen across his driveway, completely obstructing anyone passing. I hopped out and walked up the driveway. He was at work, but I talked to his wife, who was unaware that the fallen tree was blocking her in. I mentioned that I had a saw in my truck, and she was so happy to have me clear a path in and out for her and the small children to make it into town.
We live a bit out in the woods, though and you kinda have to expect stuff like this to happen from time to time.
Living in the suburbs, driving a Corolla around....not worth fighting the fumes that will inevitably creep their way out of the trunk.
I did keep a saw in the cab of my truck for a short time because I was too lazy to take it out, and damn will those fumes give you a headache before you realize it!