Lots of reasons to leave a gun in the car/truck depending on the situation of the owner and where he lives and/or works. It is a perfectly valid thing to do. We have frequent lengthy threads on what we call "truck guns" same thing.
I would have no worries about a Glocks ability to withstand sitting in a car year round. You are going to drag it out and shoot it periodically anyway, and at that point I am sure you are going to inspect it. You won't find anything wrong with it, but it is good to inspect any potential self defense weapon periodically anyway.
I keep a very small .380 in my Jeep at all times as an extra, a BUG, and as something I can slip into a pocket at a moments notice if I need to go to town at some odd hour, or after work when I have taken off the full sized pistol.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer