I guess I am the lone dissenter here who keeps gear in the car all the time. Usually a small duffle bag full of lure and plastics and two to three combos. I change them out depending on the season and what lures/styles have caught my attention at the moment. I also keep a second duffle with my waders and other cold weather fishing gear in there, my wading boots, my hiking boots, a third duffle with a full change of clothing, rain gear, and some heat/cold resistant snacks. There is usually a case of water, a pint of bourbon (hid under my jack), and a case of energy drink mix. Under the drivers seat there is a small bag of common automotive tools and a spare belt. I also keep a growler or two incase I discover a new place that fills them. In the summer I also keep my PFD, break-down paddle, and my water shoes in there too. I have a little RAV-4 and have already had to take one back seat out and I am eyeing the other one for removal.
I have never noticed any issues with my combos or tackle, but everything is out of the sun and covered with packing blankets. I am not too worried about theft as my renter's insurance covers things stolen from my car. I keep it all in there mostly out of laziness, but also because my work schedule is so weird that I like to be able to hit the water whenever I get a free moment without having to go home and load up. I am even looking at buying an old beater minivan so I can keep my kayak loaded up too, sort of a "Fishing Mobile Command Center".