This rambling is annoying when the original problem hasn’t been solved or question answered. On the other hand, if it has, I don’t see much reason to be so strict if the OP has already got enough information and the discussion starts to spread.
I also expect the OP to be active in his/her topic. Oftentimes it happens that the OP doesn’t give enough information or doesn’t respond back and the discussion spreads because of speculations leading to different directions. It’s of course annoying to those who afterwards search for solution to their problem and try to read that thread, but the OP can blame him/herself.
We also have to remember that this is a community and human communication. Part of that is that this isn’t only robotic spreading of facts and answers. Rambling is part of the nature of humanity and shouldn’t be squelched.
One example of a thread where I feel keeping on topic is especially important and where I asked that the moderators would move offtopic discussions is here: Post-v5 new features and development news. The moderators (or was it only one, Rene?) responded and I’m thankful. Anyone, especially original posters of threads, can do that if they feel their threads have been mistreated. In my opinion the original poster kind of “owns” the thread until the question has been answered.