I think in a lot of cases that would be overreacting. Conversations have a natural tendency to drift into related topics, and the line between one topic and another can be quite fuzzy (and opinions of the relevancy/connection can vary from one forum member to another). It's a judgement call.
If someone popped up in the midst of, well, any news topic here on MR, and said, "Hey everybody, what's your favorite breath mint?" and that developed into a many-page-long discussion, yes, that's way way off-topic, but, more importantly, it's very likely to detract from the main topic of the thread - someone trying to read about the stated topic would have a lot of "junk" to wade through. In that case I'd report it - again, not because it's off-topic per se, but because the volume of the off-topic posts detracts from the original conversation.
If it's a handful of posts, or it's at the tail end of days-long thread that has mostly played itself out in regards to the main topic, I think reporting it as off-topic is just wasting the mods' time. (I also wouldn't call the police if I saw someone jaywalking.)
The one case where a small number of posts gets closer to being reported is the rare occasions where Apple does - or doesn't do - something (I can think of a few examples relating to problems they weren't acknowledging, like in the early days of the butterfly keyboard fiasco), and someone decides it's their sacred duty to invade Every. Single. Other. News. Story. with righteously indignant comments about "how come Apple hasn't fixed/addressed <X>?" It doesn't matter how righteous they believe their position to be, they're essentially trying to derail every other conversation on the whole site, to talk about the thing they consider most important. That I will report.
