First of all, you can't call "BS" on something like this. "BS" would be if I said "Messenger bags always flop around and they're all crap." I've read several threads from people here who have had this problem with a messenger bag - hit a corner, and it slid around on their bike. Maybe you have one that doesn't - great. Please share. But there are definitely some that are difficult, "BS" it is not.

Second, what bike do you ride? My comments were regarding one of the twitchy road race bikes were you are rather leaned over. There's nothing I can put on my back at all that I don't notice while riding that bike. My flat bar bike is no problem. Even my more upright road bike is pretty decent. But on my leaned over, twitchy road bike anything at all on the back is noticeable the entire ride.