I'm not a moderator but I can give the writer point of view.
When I was starting, I used to post a placeholder thread saying which sets I was considering and asking for the feedback. And the latter is the most important reason: having placeholders means people will give their feedback! While I was playtesting the sets I thought were good, I often checked the thread and I have to say I got great opinions and a few made it into the analysis as new sets.
Of course if the threads are inactive, then move them to L&O. But that's not a problem of placeholders, it's a problem of every stage in C&C.