I don't think I would leave them in the car at all.
You are right of course but sometimes I have no choice, I even take her with me when I am working and people accept her willingly. During the summer they don't always like hearing her crunching treats in a corner of the room or licking their toes, but I am lucky, they stay cool about it! Everybody knows Anuby.
She only stays in the car when the weather allows her to be comfortable in it, don't worry. I am looking however for a really big crate for the house, one in which she cannot hurt herself as it's too hard to take her with me everywhere. I know she can stay alone for 5 or 10 minutes at a time but more than that, like others, she may go completely wild and throw plants about, pull curtains down, chew cables, etc, etc. But I still keep an eye on her when I am with her as (and it is getting better) if I don't see her for 5 or ten minutes, the chances are that she has found a pair of shoes to chew! Still, I got her when she was 2 years old, so training is a bit difficult now and I don't know what she went through before!