I guess I'm not understanding the logic behind "if someone wants to hurt your/rob you, they will find a way, so what difference does it make if you lock the door."
A locked door is not a foolproof protector, but it is a deterrent. A big dog isn't foolproof either, but it is a deterrent. Lots of things, like having adequate outdoor lighting, and alarm systems, neighborhood safety patrols, and not having big bushes in front of your windows are deterrents. A determined criminal can certainly circumvent them all, but at least their presence will buy you time to get out of the house, or call the police, or persuade the criminal to move on to a house that has fewer deterrents.
I could decide that if someone is determined to abduct a child, they will find a way, so there isn't any point in me walking my children to and from school, particularly when it is often inconvenient for me to do so. But I feel my presence, while again, not foolproof, is at least a deterrent. So I do it anyway.
I started this thread because I honestly could not see any benefits to having unlocked doors that would outweigh the potential risks.
Fresh air is not a tangible benefit in my case - we don't have a screen door, and the weather is so humid and breeze-free out here that opening the windows in summer is truly miserable.
Ease of going in and out is also not a tangible benefit to me. We don't do a lot of running in and out, not because we are paranoid or locked in a fortress, but because we like to hang out around our own house. When the kids play in the yard, I go with them, and no, I don't lock the door when I am sitting on the porch two feet away from said door. If we run errands or leave the house for any length of time, we lock the door, which takes me less than 30 seconds to do. My house keys are on the same ring as my car keys, so it isn't as though I have an extra burden in keeping track of them.
For those who do feel those are significant benefits, there's nothing wrong with that - it's the choice you've made after weighing the pros and cons.
I have to admit though, I really cannot understand why anyone would purposely leave doors unlocked overnight. And again - it isn't about why you don't have to - but what do you gain by not locking the door at night? It takes seconds to do, and there doesn't seem to be a downside.