If it's one thing the people on this forum must admit whether they like it more or less is that it has remained a fairly structured one. Threads being necroed left and right is certainly not something that will ensure that new and current conversations stay on focus and avoid having subjects that are long dead go on.
Now I must admit that some threads has been locked a bit too quickly in some cases that could've remained open, but the mods or people that point out not to necro is trying to help just as much as the one actually posting in an old thread, at least in most cases.
From what I've seen, the majority of the cases when the whole "necro" thing comes up, people have been posting in ways that doesn't really provide much to the thread itself or even the board, sometimes the thread is old stuff not relative anymore, sometimes the replies are short and do not bring anything to the subject, sometimes the users posting show more or less that they are new to forums and don't know how to navigate it's threads as well as a more seasoned user, sometimes it's even clear that the user is just posting to boost their postcount.
There's ways to post to avoid this really, if it's an old discussion thread maybe check up if it's even something going on still, if so, maybe go more into detail why you as a user decide to post in it and bring it back. It will look like necroing if you just say "Yeah, this is a good idea!" and leave it.
I must say though not all threads are the same, and things like mods and creations should most of the time remain open unless the original poster wants it removed or closed. I personally will keep an eye closer on the boards regarding closed "necro" threads and make sure it stays on a good line. I think the forum has faired fairly well when it comes to this matter though.
