If a thread can still be relevant, why does post date matter? The only reasoning I can possibly see for this is when people necro threads for the sake of necroing (such as posting something like 'necro necro' on 5-6 threads), in which case I would consider that spam.
~ @chaos546
Because it annoys everyone, lots of info on the threads are no longer valid/true, and some threads are just useless and don't need to be brought up again.
+1 for a rule, or automatic thread locks after a month. If you want to resurrect it, make a new one.