I can understand this disagreement from all sides.
Not using your car saves you money on fuel, repair bills etc. (and causes grief for managers)
Using your car for conveyance - yes you incur more costs (and business insurance is required (it's also necessary if you work at different locations)), however you don't have a wait of up to 1 hour for a driver to get back to the depot to take you out. Also as some people will have a journey of several miles to get to the DO their actual delivery may be just round the corner from where they live why not get the car most of the way home.
Using the car as a mobile bag drop - This is the option that I hate as it can make the car more vulnerable to crime. However as I have been left waiting for 1 1/2 hours for a bag to be dropped off (after phoning the depot twice) I can understand why some people will do this. I have also had a driver asking me why I didn't take all the bags out myself.
I have also had drivers who have refused to take packets that are too big for me to take, who have insisted that all bags for them have been ready before 08:30, and refused to take bags if management will be taking 1 bag to the same bag drop location.
No matter how much abuse / insults get thrown at people who chose to use their cars it is their choice. Many of us may not agree with this choice but we must respect their right to chose.
The quickest way to reduce the amount of cars being used would be to end the Job&Knock culture that exists. If all delivery staff had to return to the depot after delivery then what would be the point of using your car.