Here are 2 screenshots of a Gmail account in K9, first one showing 1 unread mail in the Spam folder, the second one showing the same account after the spam message has been read. That's the idea and I believe they did a very good job highlighting/showing that.
On the other hand, if I understand it right from your reply, what I was trying to suggest was that the number of unread messages should clearly be corresponding to the Spam folder, in other words it should be shown (like in the screenshot) in front of the Spam folder only, not unified with Inbox, Global or somewhere else where it would definitely be creating a whole lot of confusion.
Anyway, as always, a picture is worth a thousand words, so here you are. Thanks.