I'm curious about this as well

Same here, there was a giant tree on my land i really wanted to keep. I was rearranging dinos (and NPCs) and fly an npc a little to close to the tree. Chop. Tree gone. :(

One thing you can try is going into behavior and setting them to Follow Lowest, then have them follow you to where their class would be most useful in defense. When you get them in place, set their behavior to Aggressive and Turret Mode and set their attack range appropriate to their weapon. I usually do this with low level NPCs so they don't run off and get themselves killed.
They will sometimes move from their position to attack but usually not too far from where you placed them. They will not harvest again until you turn off Turret mode