Part of this is because we seem to have this strange relationships with media created celebrities and make emotional connections with them, even though we have no real idea what they are really like. We sort of treat them like an exhaulted family member. When we see their pictures our hearts race a little, much like they would if it were of our child or loved one.
While we do this, we promote the photographer who happened to get access to this celebrity, to a celebrity status as well, somehow thinking that since we "love" the person in the image, we are in some sort of awe of the person who took this masterpiece. This picture says nothing more about the photographers ability than the fact that she had access.
The magazine industry understands this and finds that readership goes up with virtually every magazine on the racks, if a celebrity is on the cover. This happens even though there is nothing about the celeb in the magazine. It's an strange interesting human quirk.
The adage "if you want to be a famous photographer, take pictures of famous people" applies to this genre.
This picture was probably during the time Leibovitz worked for Rolling Stone and had access to rock stars, and almost any picture of one's rock "hero" was considered sacred.
I agree this is a "nice" picture of an unglamorous moment, but without the emotional 'baggage" that you, a lover of John Lennon bring to it, is it a good picture?
In my opinion it's not a bad picture, just nothing special. Maybe to you that is the point. Not sure.