I agree that "locks keep honest people honest" is a nice but bogus sentiment. Bike locks are a clear counterexample. While a cheap cable lock will keep honest people honest, your bike will soon get stolen. A Kryptonite-style lock is kind of the minimum. But even those need constant upgrades as bad guys figure out how to pick them or freeze them with canned air. Thus, the goal of a lock is to make it not worth the effort for a thief, not to keep honest people honest.

That and to provide evidence of forcible entry for insurance companies to pay out. It's easier to cut the lock off than to learn to pick most locks, so the victim can easily prove the theft.

I don't know if this is actually true, but I've heard that it's partly about taking away plausible deniability (for people caught on another person's property). If you had to break something/pick a lock/climb through a window, it's hard to argue that you had no malicious intent and were merely trying to pay a visit, or that it was dark and you were a little bit drunk and you opened your neighbour's front door by mistake.

That reminds me of the time growing up that we actually had someone astonishingly drunk who ended up falling asleep in our unlocked minivan because he completely screwed up navigating home and apparently we had the same type of van he did. Mom was not at all amused at the time, but the more I think about the implementation of car locks (most share key cuts), the more I find the entire thing somewhat amusing in hindsight.

I think the saying "locks keep honest people out" means that locks are there to keep out the overzealous girlscout selling cookies, that annoying friend who comes over when you dont text him back, and your neighbor that heard a noise and wants to make sure everything is ok.

It also deters some dishonest people. A while ago my suburb was hit by a drug addict who moved in and would steal change/ipods from unlocked cars and tools from unlocked sheds. He never touched anything that was locked because the extra risk and effort wasn't worth it.