I've changed this option back-n-forth several times & don't notice any difference. I have noticed that it seems that old messages are only retrieved when there aren't any new ones. The times when there are new messages it'll only retrieve & report the new one(s). I had left the timeline contact open overnight & there were over 300 new messages, but I could see that they were the same block of old ones, retrieved over and over again. So it doesn't make a difference if I clear the current messages or not.
And yes, the author is very invisible here and doesn't seem to spend too much time here helping with the issues.