> the downside of this feature is my wife's car does not have it -- and so at least half of the time when I am driving it I forget and leave it unlocked in parking lots.

My brother in law did this on a ski trip with a borrowed Range Rover. It was only at the end of the week he realised he'd left his keys in a jacket pocket in the car the entire time and it had been sitting unlocked in the car park half a mile down the road from the apartment. Thankfully it was fine but stealing it would've been a case of getting in, pressing the start button and driving away.