I leave my cues in the car year round. Never know when I'm going to be going to the pool hall. To me, being prepared is more important than babying a cue. But everyone is different on this issue.
The risk is warpage, theft, and inlays moving damaging the finish. But I know many excellent players with high end equipment leaving cues in their car.
I guess it's just my OCD. My 10 year old Mezz still looks basically new and I play 2-3 nights a week.