Its called Necro posting...its kinda like posting in all caps...its something you dont do. A lot of the time its something thats irrelevant just to get your post count up like you do a lot of the time. Some of the time its relevant to the new person but its a waste of bandwith and clutters things up by bringing back a very old thread.
Like this thread for example is almost a year untouched and the guys says "This is great! i was thinking about making mine like this but wussed out and went with fiberglass, but this is gonna be sick! keep it up. " What does that have to do with anything...maybe the poster finished it without posting it up or started another thread with the completed version. Either way theres no reason to bring back the dead.