Krismo962
Necroing a thread is generally frowned on in most forums. The reason is it buries more recent threads. Imagine if everyone necroed threads and you posted a thread and needed a answer or where sharing something you learned that might help the community and it never got an answer/response because it got buried under a bunch of old threads that people necroed. It might frustrate you and if you're a new user might make you not return to the forum. Especially if it's a common place thing. And if you where sharing something helpful the whole community would lose out because dead threads made your info less visible.
Now if a thread was brought back because there was new info. or a update that's different but even then it's usually better to create a new thread. You could summarize your last thread and include the new info. to make it more easily digestible for the readers. Making it a more effective means of passing info. Just saying what's up or referencing something humorous from a old thread is better done via PM or in the person you wish to address new thread but that could lead to derailing the thread, which again is counterproductive and derailing is against TOS.
I know it's not obvious, as necroing would seem to be innocuous to most people, myself included till I saw the effects for myself.