Whew - I am late to this thread - but I read most of it...
Well, I had an immediate bad reaction to the OP because my DH was locked in his room by his mom until a very late age, like 7 or 8 years old, and would scream to get out to pee, but Mom would ignore him. He ended up having to pee in the heating vent because she wouldn't let him out. Yeah, he's not a big fan of his Mom.
My earliest memory is having a nightmare and crying and crying but no one ever came to comfort me. That stuff stays with you I guess.
After reading this though, I can totally understand that there's a big difference between that (CIO, basically) and the act of locking your kid in a room for safety reasons. My LO is still in a cosleeper, though she'll soon move to a crib; for safety reasons. At some point after that, she'll move to her own room. And then when she gets big enough to climb out of the crib, to a bed. If at that point she starts going on middle-of-the-night adventures, I could totally see locking the room she's in, but NEVER ignoring her. Luckily, her future room has both a door to the hallway as well as a door directly into our room, so we could probably just lock the one to the hallway and she could come bug us directly