You have to realize that a wing of 4 players cannot deploy additional NPC wing members.
If you have your 3 NPCs deployed and you're being invited to a full player wing, your NPCs should despawn.
We are talking about wing members, not NPC pets. A wing is limited to 4 slots.
This makes sense. Join a player wing, NPCs despawn. That would prevent overloading an instance with ships and prevent uber-gank squads.
As far as NPCs becoming combat slaves while the player parks 10km away and gets a sammich, if the player becomes inactive (doesn't fire his guns at something or mining laser at a rock or make some sort of mission progress) then NPC wingmates stop what theyre doing....
NPC wingmates shouldn't fight to the death for some idiot player commander sitting behind his desk being lazy.