Some off-topic tangents are inevitable in many and perhaps the vast majority of threads. It is just the nature of human discourse, and discussion and critical argument should never be discouraged, in my opinion, in and of themselves.
At the same time, it is reasonable to expect a modicum of respect for the original poster's intent and wishes in creating their topic, and off-topic posts should be kept to a minimum whenever possible. When a moderator feels the need to step in and remind members of this, I think it wise to emphasize that the worth of the discussion in and of itself is not at issue, but rather the placement of such discussion. New threads should be encouraged to preserve and foster discussion that has lost its pertinence in the threads in which it first evolved. I applaud the existing record of our moderators in doing precisely this.