I will disagree with this. Clicking on the bar has been part of the interface for some subreddits for a while now (though not many). I find it quite nice because if you are reading a deep thread and want to collapse to the next N levels up, then you don't have to carefully scroll up while watching the vertical bar to find the top comment in order to minimize that thread. This makes it very easy to skip to the next top-level comment when a topic has lots of comments.
I think the way it works is nice, and what you described is a good use for it. It could use some better UI to make it easier to discover. It didn't occur to me to hover over or click it because I thought it was merely an indicator.
