I don't think that many understand the real beauty of longplays (unlike you and me).
When watching a longplay, just lean back and enjoy the show.
You are going to see everything that you don't see when playing the game. All the details you never noticed was there because you had to focus on the actual playing. What's more? A longplay gives you the feeling of the old times and all the memories comes back when watching a whole game from start to end. And when someone real skilled play you are going to see every secret on every level. A guy played through both Streetfighter Alpha 1, 2 and 3 without getting hitted even once and managed to won all the rounds with a perfect in all three games. That's skill.
I admit that something similar to RZX (with rollback) would be nice to see on Atari too.