Well its to be assumed that the purpose of the topic is to discuss the mentionings in the first post, made by the topic maker, and thus saying not to take it off topic is redundant, but then again, off topicness should almost be expected, if you want people to continue to post. Not to mention certain posts remind us of stories or comments we'd like to share. It just happens o.o
Its like letting children loose in a playground and asking them to only play on the jungle gym, then leaving. When you come back, there is little to no chance children will have only stayed on the jungle gym.