Well, what for ... the message numbers are useful in long threads (in flat mode) where a small group of people contribute. In threaded mode it is easy to see to which message someone responds, but in flat mode it is impossible if the only information provided is a name. My W3T forums are not very old yet, but the community using them now is a few years old -- we have threads that go over 60 default pages (sigh, this is in only three days) ... most of those users prefer flat mode, and they asked about the missing message numbers.
I'd be happy if this was made an option. If you don't need it for your boards, you won't have to use it -- but there are communities that would benefit from it (personally I find message numbers more useful than the new user list, but it's all about preference).
-M.
