I don't think rucksack is better than messenger bags.
I have a lot of bags (too many, really) and to go to work I prefer messenger bags because :
- I wear a suit at work, so I need to fold it into my bag and I find it's more convenient to put it in a messenger bag without making ''creases'' thanks to the wider opening.
- I'm short and rucksacks with large capacity (40 liters or more) are often quite long. Hence they are too high and disturb me when I raise my head on a bike.
- You can swing a messenger bag when you need to look for something in without having to put it down on the ground.
Rucksacks a more comfortable to carry heavy loads, but professional messenger bags have a optional/additional second strap which goes on the second shoulder (I don't talk about the thin strap which prevent the bag from sliding).
Thanks to their wide opening, messenger bags also allow to carry parcells which wouldn't fit into a rucksack. For instance I carried a vacuum cleaner in a messenger bag, I couldn't put it inside a rucksack. I wish I could have, because I would have preferred to carry 17kg with a rucksack !
NB : I talk here about real messenger bags, not look like bags. For instance I don't consider the North Face bag as a real messenger bag. In fact it has nothing to do with a messenger bag.
(sorry if the words I chose are not correct, I don't speak English so well)