Perhaps I can provide some more information on why it is useful. Also if few requested it perhaps they never operated a board that has it before. Sometimes you don't know what you got until it's gone as they say :-)
My moderators had this feature on VB2 which did not have parent child. They loved it. After the switch to aspg they were all extremely frustrated at the loss of this important tool which is used to provide better order in the forums and threads. VB3 which has parent/child relationships still supports merge/split.
Why is it important? Let's look at an example or two.
First let's say you have a 50 or 100 post thread. It then gets "hijacked" which means off topic in some way. Perhaps the new tangent runs on for 5 or 20 posts. Maybe these posts are relevant to another existing thread so we would take these threads and simply move them into the appropriate thread topic. This left the original thread "on topic" throughout plus organized the tangent info into an applicable thread. This was used literally on a daily basis to some degree. This was an example of "split functionality"
For merge let's say you have 5 threads which started about the same time in either separate threads and/or separate forums. The bad thing to do is let them all hang out there on their own. It isn't efficient, tidy or logical. We would take them all and merge them into one concise thread to keep the information together.
So if you asked my 50 moderators if they felt it was important you would get a resounding yes from all of them. As far as what priority to include this functionality I guess that depends on two things. How complex a merge/split function is to implement in the code and also what the importance of other items in the "to do queue" are relative to it.
I hope these examples better elucidate why the merge/split is very useful in an active forum.
Thanks as always for listening.
marc