I know sometimes when I see a contradiction between an old post and a current one, it's because I had a vague memory of a previous thread that was relevant and thought I'd look back to see what was said. Sometimes it changes my opinion, and sometimes I see that as useful and post it.
I probably do the 3 item on Shore's list. I don't expect people to remember things about me, so if there is something out of the norm for me that gives me a different perspective, I post it as a reminder. My using a chair is on that list for me. It's changed my perspective, so I bring it up. I never mind people giving the context which forms their perspective, even if I've heard they were pregnant for a while now, until threads get twisted around one topic every time a poster shows up.
I think bringing things in from other threads is useful, but there should be some care not to derail the current thread. One thing that happens is when we have thread A and thread B dealing with similar circumstances. I'd rather see a spin-off discussing why people feel one way in thread A and another in thread B than see either of those threads derail because we were trying to compare within a thread.
Context in terms of old posts can be helpful. Recently there was another post by Yarnspinner (sorry, I'm borrowing your thread because it was pretty relevant) where a new person didn't know the background with Stonecold. I made the critical error (
)of trying to gather the past threads up a bit and give the context. A full page of threads later I think I may have scared Yarnspinner. But, if it was just that one incident, it didn't seem so outrageous.