I believe one of the reasons why some members don't like it, they get all excited a new thread is made on something they are interested in, only to find, it's a dead thread, except for one recent post just made which rarely adds anything of substance after that long of a period of time. What really gets them up in arms is if a newbie does it who just adds something like "oh cool, I like this."
Another issue is the help me troll who frequents this forum from time to time who does it because he likes being a pest, I think it is the same guy who keeps making new member names and posts medical questions about being hit in the eye by a laser beam, although some legit, I am sure.
Other than that, I don't care if someone necro posts, better than making another thread about the same thing, as long as what you are adding is a reasonable question, or continuation of information improving the thread. What I mean by reasonable question is it makes sense, stupid questions are also fine to me, I make them. Although, we probably ought to have a stupid question section so people can feel free to post anything stupid they want without fear of someone whipping them with a wet noodle, as happens. The whole forum should be that way, but it isn't.
