If the lock is missing when you pick up your bag, you would be alerted right then and there to inspect the contents of your bag before going to customs. (Though if the lock is something with a commonly available key that allows opening and relocking without indication of being opened, you might still not be able to notice whether your bag has been opened.)
So what if you are alerted right when you see the bag? Are you going to pitch the drugs in the garbage can? Explain to the customs agent that they really are not your drugs? How is s/he to know you did not just panic and get cold feet?
I use colored cable ties on the rare occasions I check a bag. Cheap, simple and will encourage the average thief to grab the next bag.
On life's list of risks, becoming an unknowing drug mule is pretty low on the list of things for me to worry about.