As ChAiNz said, there may be various reasons to post in an old thread and some posts can sometimes add something useful and revive a good discussion. A few other examples would be reviving an old tech help thread when the poster experiences issues similar to what has already been posted or posting in an old modding thread that discusses a specific issue (we aren't going to blame people for using the search function in such cases!). Sometimes a post can add something useful to an old but interesting general topic and revive a good discussion.
However, things like reviving old threads just for the sake of annoyance or posting one liners like "omg yeah me 2!" at the end of a 2yr old thread falls within the trolling/spamming category. It's a case by case situation and we already have rules to cover the unwanted situations.