There was one point in time on AFF where the opposite was kind of being complained about - moderators (or particular moderators) appearing to be overzealous in either deleting off-topic posts or fervently splitting them off to new threads. The main thrust of those claiming overzealous behaviour was that whilst the posts were not strictly on topic, they added colour or qualification to the main line thread, or in other cases the degree of distraction of the "off-topic" posts were not as significant to warrant the moderator's actions. I say this carefully, of course, because disputing a moderator's action is against the ToS.
I don't know if we have the balance right or ever did, but I have little problem with the current way things are going. Internet forums are always going to be a case of stop-and-go reading (unless it is one of those special threads with solid material without interdispersed commentary, then locked). I agree with admin that reporting a post (in a set of posts, as the case may be) if a thread appears to be running off the rails would be the way to go, and then moderators will delete or split off the off-topic posts as necessary.
Overall, it is not as if moderators do not already have the power being appealed by the OP. They can delete any posts that they want; reasons do not have to be provided and requesting one from a moderator could result in a warning or disciplinary action anyway. Moderators do have to give a categorical reason if you are served a warning or suspension (as I understand it, because the forum system makes them at least provide some sort of category which is communicated to the offender, e.g. bullying, spam, etc.).
I will admit I am guilty of this as well, but often I will flag my own post (e.g. prefacing it with "This is probably off topic, but...") as potentially diverting off topic, if that means it will even flag to a moderator that the post might be removed or split off.