As much as I love taking my puppies with me I always leave them home on hot days unless they can get out of the car. If you leave the car running there is always the chance of your pet bumping into the gearshift and causing an accident. Not to mention someone walking by and being tempted to steal an unoccupied running vehicle. (not all car thieves are afraid of dogs)
Also, I believe in some states there are laws against that (leaving pets unsupervised in vehicles as well as leaving a car running unsupervised).
As far as your dog being "nothing fancy" I bet he is better than the 'fanciest' of all dogs to you and that is all that matters.
Jihad
and the pound puppy crew.