When I have filters on, there are times when I have some number of unread messages, none of which I can click on to clear the unread count because they are filtered.
Currently, I toggle filters off, use all my willpower not to read anything, click the last one and then toggle filters back on.
Having a little "Mark All Read" button would give me a quick solution when I noticed this issue and I could stop obsessively looking for that 1 unread message.