This is only true if you assume users always scroll down while reading and the only reason they scroll up is to find the header... but many of us scroll up and down while reading and find the re-appearance of the header to interfere with our goal of reading the content. So there is a clear downside for us "up and down" readers.
I don't know what portion of users we are though, I'm glad to see I'm not the only one!
I don't know what portion of users we are though, I'm glad to see I'm not the only one!