Alright, so you want reasons not to, well, I can't speak for everyone on the board, or the administration's reasons, but these reasons are why I don't particularily like it.
If there's a dumb necro like "can't wait to see it in game" which adds absolutely nothing to the thread whatsoever and just dredges up something that might be entirely irrelevant to the current state of affairs up again, it's better off just not saying it.
Even if you have something to add to the discussion, if it's a discussion that's been dead for four years, chances are everyone has said what they wanted to say. Dredging it up again just because you happened to miss the discussion while it was taking place just makes it seem like some sort of attention whoring thing - they stopped talking about it four years ago, but WHAT I HAVE TO SAY IS SO BRILLIANT everyone will suddenly care again. At least, that's partially the impression it gives. Chances are if it's that old, the discussion has concluded - everyone has forgotten about it, or moved on. If there's really something new worth discussing then you could make a new thread for it. I think having multiple threads on the same thing isn't a bad thing per-se, as long as the others are dead. It's certainly less awkward than opening up a thread to see a post you made four years ago being replied to...which is like a "wut" moment. Half the time the discussion isn't even pertinent to the title anymore.
Even if it is a Q/A thread, I'd still just open a new thread and make reference to any previous ones that may be related. Keep issues separate, as they might not be as similar as one might initially believe...
I don't see anything wrong with wanting to participate but, "there's a time and place for everything", sometimes that time is past. If you feel there's a new development that warrants renewed discussion, then make a new thread. If there isn't really and you're just responding to something because it's there, then I think timing as a lot to do with it.