How rigid is the off-topic rule? If a thread has mutated into a discussion of something very different yet still retains merit is it acceptable? I find that unplanned drifts in threads are often the most interesting. Consider this thread. Its been off-topic since halfway down page one yet not in a bad way.
I usually try to relate what I post to the OP in some tangential way even if I'm really talking about something completely different. Should I change my ways?