I think the Subject should reflect the comment. If the topic of the discussion or the focus has changed, then the Subject can help indicate where and why.
The best Subjects help readers understand the main point the author wants to focus on, but I also think it's reasonable to set up a joke with a Subject. I think the vacuous Subjects basically encourage people like you to devalue the entire idea of using Subjects in constructive ways to help readers. Call it a kind of lazy writing, though I think it's often a symptom of a lust for FP. However I think it's actually best to start with a Subject that will guide my writing and then look at it again after I'm done to see if I've lived up to the Subject. If I do have to change the Subject at the end, then I should think why.
I think the worst use of Subjects is as a kind of marker to "claim" ownership of a thread. Vacuous Subjects seem to work especially well for some identities and non-identities.
Then again, it depends on why you're writing. One of my reasons is to try to write better, and the results to date do not make me feel happy. However in terms of clarifying my own thinking about a topic I sometimes feel like I make a bit of progress as I struggle to get my thoughts in some semblance of proper order.