I guess until I try a fanny pack riding, I'll never really know, but from never having things on my back or hips riding, I suspect I won't like the weight and or movement that would be felt with moving around on the bike--movement that decades of riding has me used to the feel of nothing there....
guess I'll just have to grab the fanny pack and try it one day with different weights and objects in it to really see......
Where I like fanny packs for hiking, cross county skiing, snowshoeing, I am upright. If I am leaning forward more like my posture on a bike, I would not want a tight belt strap around my stomach.
I think that the really skinny people are more inclined to like a fanny pack on a bike. The friend of mine that always used one, he was pretty skinny.