
I think they're absolutely essential, and it's the first thing I buy for a new car (and a padlock for the tailgate). Not only is it safer, if you're opening your boot door in a busy area say, stops the dogs jumping out until you are ready. They are essential if you leave your dogs in the car, even for a few minutes on a hot day. If I stop at the shops for a paper I still open the boot door so the dogs have plenty air, more secure than leaving the windows open and risking someone entering or even stealing your car. Invaluable at open shows/days out where you can sit beside your car, or go to the loo and know your dogs are secure and cool. If you're going out for the day and want to stop for lunch you can leave the dogs in the car without worrying about them. Also in the event of a rear end collision I've heard of boot doors springing open, the tailgate might bend but would be unlikely to open. They take minutes to fit, most are completely silent when travelling, most are easy to remove if you need too. There are lots and lots of good reasons for getting a tailgate and only 2 reasons for not getting one - money and the screw holes in the car.
Edited to add you can lock your vehicle with the boot door open, either leaving the boot fully open and simply closing the boot lock (so the central locking thinks the door is shut) or by using a ventlock. I have a long ventlock, 24", and like it when it's raining as it's holds the door open enough for air but stops most of the rain. In warmer weather though I prefer to leave the boot door fully open and padlock the tailgate.