My personal rule of thumb for when a topic is not worth participating in:
If half of the posters feel strongly that X, and take it kind of personally that others believe NOT X,
and half of the posters feel strongly that Y, and take it kind of personally that others believe NOT Y,
then it's really not a productive conversation to continue having.
I sort of feel this way about throwing your own birthday party threads, to give an example. Some people think it's the height of classlessness. Others think that it's completely normal. These threads can often get a little personal (comments like "Why would an adult feel the need to have a birthday party?") So what's the point in having an ongoing discussion? I could see posting once and saying your piece. But commenting over and over isn't going to change anybody's mind, so why bother.
So for the majority of the threads mentioned here - including the childproofing threads - I don't find them to be no longer worthy of discussion. I think some threads get kind of circular, or sometimes particular posters become a little too attached, but that doesn't mean we can't try again later.