Everyone has their own thoughts on merging. As a moderator on another site, I know that even within the mod team, there are opposing views. There are obvious topics which need to be contained within one thread- you all know within your favourite forum which these are. A pm to the original poster where the merge would not be obvious is a good idea in an ideal world. However, we have to remember that mods are often also in full time jobs and some days just do not have the time to do everything they would like to do.
Where a thread becomes too lengthy a suggestion would be to start a fresh thread and leave a link to the original one on the first post.