It's akin to locking your car in the parking lot. It makes clear you don't wish anybody to get in.

Actually I think Inside-Out objects are more like a car alarm system. If you try and mess with it, it will likely complain very loudly but ultimately it can be bypassed by the determined.

IMO, simply using accessors is "locking your car" because OO Best Practices already dictates that you should always use accessors to get at your slot data.