I will say that I disagree w/having the doors opened and/or unlocked but thats just my opinion and my way of doing things. many people might have been doing the opposite for yrs, and it might be working for them. It is a habit for me to usually lock my car door when I am driving too.
It is known that many criminal acts happen due to the fact that a window is open, a door is open, or said doors/windows are unlocked. Many, many criminals have confirmed this during interviews of their specific crimes, so its not just statistics. Also, many people don't CCW because of the statistics on their side(if the statistics demanded more CCW, more citizens would utilize the right--as in they don't feel its necessary to have a weapon everyday for defense).
Many people have perished due to home invasion or have been raped and/or abducted due to the fact that their window or door was open and the neighbors' windows/doors were locked(from multiple convicted murderers confessions they just go until they find that is accessible). I believe the night stalker in the 80's used to cut thru screens. The light that I pay 7bucks for extra in my electricity bill(its the company's light and was already up when we bought the home), the alarm, the firearm, and keeping the doors+windows locked are all preventative measures but most cops will tell you it helps. One can be surprised so much more easily if the entranceway is unlocked, and someone can also lie in wait for you. I could probably leave my doors unlocked where we live, and possibly for us its just a habit or some kind of 'paranoia' as to why we leave them locked, but I wouldn't feel comfortable doing it+I believe its a needed deterrant. If my Grandpa(may he rest in peace) was here, I could probably have a losing debate w/him about the issue.
On January 1st, 2006 a richmond couple and their two children were killed and robbed after being tied up. The criminals after being caught explained that the only reason they went to that house is because the door was open(the couple had been expecting to have a new yrs day party). The mother actually answered the door while the intruders were in the house and didn't alert the other mom who was dropping her daughter off from a sleepover that something was wrong. She was coerced by one of the intruders and mistakenly thought that as long as she did what the intruders wanted to that noone would be hurt. The 2 men slashed and killed all four and burned the home down for some small amount of money. I mentioned the paranoia thing and do one more time because it has come up in this thread; growing up I didn't worry about it, but now I do since I have my own home and family. I have seen too many horror stories on tv or in the news. Maybe it is paranoia, but I do not think so.