I tend to think that if people are actually having a fun, productive discussion, it’s a waste to close the thread, even if people had a similar discussion six months ago. It’s one thing when there are multiple active threads on the same topic at once, or if the topic is a pure “What’s this thing I didn’t bother to search for?” By the time a real discussion has started, though, it’s probably best to let that discussion go. Necroing an ancient thread so that people who weren’t around for the original can talk about it is… meh.

TL;DR: There’s few enough interesting active discussions, might as well take what we can get. If several people are interested, it can’t hurt that much to leave it be.