This is useful for letting you know exactly how many new posts there are: Unread Post Count In case there's confusion, the phrase What's New? was changed to New Posts in XenForo 1.2.
Besides that, it's annoying to have that little counter there (for me) when I've read all the messages I care to read.
I never actually view the new posts anyway. I more or less scan the forum for any interesting topics I want to post in. I can see how this would be useful to some but I don't think it's worth installing an add-on for.
I forgot to reply to this because I do the same thing, but in the cases where you go through everything you're interested in, but there are still unread messages remaining, if the unread message count indicator from the add-on would bother you in those cases, then you could just mark all forums read.
