That makes a bit more sense and even though I wouldn't use it (I think) it would be nice if the feature was added. Everything to make users be able to view the forums the way they want to.
Actually, I would like to have a link from postlist to view a thread in flat mode even though I usually browse in threaded mode. Sometimes I only want to take a quick look at the posts and this would spare one click. Right now I have to click on the post and then on the flat mode link.