I close my doors because I don't want bugs and other things getting in. I think the lights go off after 15 minutes, that's a pretty long duration, plus don't be surprised to get the low battery warning which everyone seems to get anyways.
One time my wife accidentally forgot to close the sliding door and i was so worried because it was like 4-5hrs duration.. Well, good thing the battery didn't die .. I dont know why, I didn't even have the 'low batt warning'..
Phil, I think 15 mins is too long. We unload some of our stuff on vacation and all lights went-off (IIRC) for like 5 mins max..
My other car in the garage is a convertible, its top is always down, and so far there is no animal / bug problem.
The main reason I don't close the doors is to save a close and open cycle on both sliding doors. I imagine 4 open / close actions use more or less the same amt of power to turn on 15 mins of interior light. Plus i don't have to wait and fiddle w/ the keys when I get to the car when I leave home.