I have also used the mini shackle to lock the crank arm to the chainstay on some bikes in addition to using the shackle on the rear wheel... my biggest theft risk is from folks who are looking for a bike they can steal easily and ride away on.

The best locking strategy is to lock up next to a nicer bike that only has a cable lock on it...

One shackle is enough of a deterrent for opportunistic thieves but I would prefer not to have my parts stripped off my bike.