I don't like the way the pages look when the headers shrink. I kind of feel naked without it. I am used to seeing my avatar in the top left. It also removes the Watchlist, a feature I often use to check for new replies in my own threads and posts in discussions I have previously participated in that are of interest to me.
You know your Watchlist is in the shrinked space so whenever you need you can just toggle the collapsed button. It's basically a personal likeliness. I see it looks good when the area is shrinked. The header part is much simpler.