I've been thinking about this and i've talked to some great UI designers...
And we got all the same minds (OK probably 90% of the users here will disagree, because they have only categories and forums) but:
1. "Forums" in navbar is also a strange name for this...
You're getting a node tree with several content types and not only forums.
e.g => If somebodies linking pages, gallery nodes, lexicon nodes,... in the tree(that's why i said it's not only forums)
2. Avarage joe sees gallerynodes, Pages, etc.. in the node tree but if they have also a "mark read" feature, "mark all forums read" won't work for them(so they'll ask why it's not working for albums, i'm seeing it in the forum tree).
They'll need to click on several links, to mark really everything read...