Threads frequently have 20+ replies, so you end up with something looking like the following:
Help me
RE: Help me
RE: Help me
RE: RE: Help me
RE: RE: RE: Help me
RE: RE: RE: RE: Help me
RE: RE: RE: RE: RE: Help me
RE: Help me
RE: RE: Help me
RE: RE: RE: Help me
RE: RE: RE: RE: Help me
RE: Help me
RE: RE: Help me
RE: RE: Help me
RE: RE: RE: Help me
RE: Help me
RE: RE: Help me
RE: RE: Help me
Now, if I wanted to go back to a specific (unbookmarked) post in that thread to refresh my memory on something that was said I have 20+ posts to review, again. If there are unique subjects, though, I can narrow the field dramatically and potentially only review a couple posts before finding the right one. With unique subjects I can also usually summarize what each subthread is about, what topics/suggestions have come up, etc. In the off-topic forums, in wishlists/bug threads, etc this can be immensely useful so that I don't have to review potentially 200 posts before being able to determine if the point has already been made.
In short, there are some who dislike having to enter a subject, but those who prefer a Tree layout instead of a flat view find them extremely beneficial and would shy away from a system where every subject is just another RE in front of the previous.
John
P.S. As to the mark all as read for an entire forum, I too would like that one to be added, but with limited resources pointing in this direction only emergency fixes are in the forums' near future.
What a pain in the ***, why can't i just have RE:... Also is it possible to have a button to mark all forums read and not juse each one?

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