I thought I had a predator-proof run and was actually considering leaving their pop door open all night so they could get out earlier than I usually let them out. Then a coon got in their run in the evening and killed a couple. Even though we have reinforced the run, there's no way I will leave that door open at night. Coons seem to be one of the worst killers of chickens, and they are nocturnal, and could wipe out everything overnight, while I slept.
Nothing got into my run for 5 years, so I thought it was safe.
So I think it boils down to the level of risk you can live with. We all have such varied situations/environments/, etc., and we can't assume everyone has the same factors as we do, so it really has to be up to you and how safe you think it is to leave it open.
I tend to be a worst-case-scenario person, so I lock them in at night.
Plus.....I'm afraid I'd get lazy if I didn't have to go out there every morning and evening. it keeps me more in touch with them.