In 1976 moved into a house in a very nice part of East London. Within weeks we had lost (put away carefully - who knows) the keys to the kitchen door. This could be easily accessed via a path up the side of the house. Gate there was kept closed (to protect dog from getting out), but never had a lock on it.
As the years progresses, I really enjoyed that open door. Friends calling round, would go round the back if I did not answer the front door, and I would often arrive back from shopping, dropping off/collecting children to find a friend or two happily making coffee in my kitchen.
On the occasions I went out without my keys, knew I could always get back into the house without any hassle. As children grew into teenagers, was easier to tell them to enter that way rather than give them keys, which they always lost.
Never had any problem with this - moved out in 2003. My next house was a terrace - in a quiet cul-de-sac. Must admit to the number of times I would go to bed leaving the front door 'on the catch', and the windows of my car wide open on my front drive.
Live in a flat now, very secure - but, often, during the day, leave the front door here on the catch, as I pop in and out around the flats, etc. My daughter who lives close by, has keys to get into flats and my own front door, often turns up unannouced, with a child or two in tow - as she comes through my front door, she always gives a slight knock just to warn me she is coming in.