I know of two instances where internal locks on new firearms have malfunctioned and prevented the firearm from working.
The first is related to the lock on, I believe, the Remington 870. John Farman reports in his "Quips and Comments" section on his website of an instance where the lock engaged itself while firing the gun. IIRC, this occurred under recoil and the gun was non-functional. I believe the gun actually needed to be repaired, not just unlocked, but I could be remembering it wrong.
The other incident I've seen reported involved a S&W revolver. A small internal spring that is part of the lock broke (apparently bad from the factory) and the lock would spontaneously engage as the broken spring was no longer able to keep one of the parts of the lock from contacting another part. The gun needed to be repaired.
Granted, I did not experience either of these malfunctions firsthand, but I trust the reliability of the sources in both cases. It doesn't have to happen to me personally to learn from the experiences of others.
For me, if it is a possible defensive weapon, I do NOT want ANYTHING extra added that could compromise reliability or functionality at a critical moment. It might be different for say, a dedicated .22 target pistol, but reliability is THE most important element for defensive firearms.
Think of it this way: We all know of incidents where guns have not functioned correctly because of broken or worn parts, or bad manufacturing standards, etc. And that's just with "normal" no-lock guns. Why add parts to borrow trouble?
I do understand the legal and political climate that is making locks more common. I've just decided to buy as many old, no-lock, guns as I can while they are still available. I may have to pass on the "Wonder-gun" of the future, but I'm sure my needs can be met by the guns I have selected.