Imagine a case.. You click to view your unread posts in a topic. You see that you are e.g. in page 5 and totally there are 7 pages in the topic. From the first post of page 5 and on, all posts are unread by you.
So you read the first post but something happens and you have to quit reading-close computer-leave....
The next time you try to see unreads in that topic, your unreads will start from page 6. This is justified, since phpbb cannot guess whether you have read all the posts in that page. So in a 25-post page, having read only the first post, left and returned after a while, you loose 24 unread posts.
To prevent this from happening, a link to mark a post as unread is needed. Having that, if you must leave, you can mark a post as unread (e.g. the first one you did not have time to read) and the next time you return and click to see the unreads on that topic, from that post you marked and on, all posts appear unread. This way you (we) will not loose the unread posts we have.
This would be a great addition and in my opinion really needed..