The thing is that when reading postings one by one, you often want to get an overview, without marking a lot of things as read.

So when you read by thread, you have to read the thread until you get bored of it, and then hide and unhide it.

What about a button "Mark these postings as read (and view next page of the thread if there is one)"? In that way you can both look around in a thread without messing things up and easily mark it as read while you're doing it.