Well, looking back, it would help out during when the forum is more than usually active because the recent posts list would not be dominated by one topic - i.e. when I have a one-on-one conversation with someone in a topic I made, it dominates the recent posts list. This is bad in that other topics do not get the attention, but in some cases like the one I alluded to, it gets attention to the topic.
Having more ten topics shown on that list might lead to some more posting, but even with five, there are times when it appears that the forum is a ghost town. Increasing the number of topics shown would make us look like a ghost village. Synchronizing it on the site would make the site seem a little awkward. Encouraging people to look around rather than just to focus their attention on what people deem to be currently popular leads to more activity in other areas.
What I am getting at is that in my mind it doesn't add enough to make the change worth it, I could be wrong, but meh, well yeah, I tend to be, but so be it.