I don’t remember if we’ve ever discussed this here.
Rule #1 in any book: If there is a door which doesn’t open, and a valuable tool [Sirius’ knife] is destroyed attempting to open it, you can be sure that at some point, you are going to come back and enter this door. No door is created just to not open.
Why didn’t we ever get to see what happens in the Locked Room? I thought, when at the end of #5 Dumbledore explains the prophecy, that we were guaranteed a visit back there. The Power the Dark Lord Knows Not = Love. There is a Room of Love which remains a mystery. I was sure that the last battle – the climax of the series where Harry wins – would take place back in the Department of Mysteries.
Shame!