How many times have you been told when picking up a unit that "it is parked **** and the keys are *****. would you do the same with your 10 or 30k car?.
Done it a couple of times on agy with rigids, if I ever need to move house I don’t need to hire a truck I just nip to a wee village outside hudds and borrow one, no-one would ever know if done over the weekend.
usually leave it unlocked. but we don’t have very much real crime round here.
done it loads of time but mind you never owned a car over 2 grand to worry about it 
You could leave our trucks in Toxteth (other dodgy areas are available) with the lights on and the engine running, nobody would steal them as they stink so much.
And the joyriders have already done their worst.