The way in which topics sometimes get derailed or post end up "off-topic" is in part, as far as I can see, a natural part of how different people understand/approach/perceive different topics.
That will inevitably force some topic drift as Joe sees X as it relates to Y, but John sees X as unrelated to Y, but Z is related, etc.
In discussions where basic principles, like censorship, are core, these kinds of drifts will be more natural as people try to express fundamental beliefs (metaphysical beliefs). This can cause a thread to very quickly drift into irrelevance for many people very quickly as the distance between ideas can be very great.
Those discussions that are more focused, e.g. a particular piece of file renaming software, are less likely to suffer from that broad drift, though they do tend to drift based on feature sets and related issues, which may or may not be useful to the discussion.
I don't really have much of a problem with drifting topics - it doesn't particularly bother me. I suppose that I just expect it.
Then there's Stephen...
(jk - Couldn't resist that one~!