^^^^ Or you can get something like this for those situations and have the best of both worlds:

http://www.bikebagshop.com/ortlieb-v...er-p-1720.html

It does have some things I'm not wild about...which is partially the nature of compromise/do it all products...but it does address these issues. I found this very useful when I was carrying heavy books to a class that also entailed some walking on campus. I think the QR3 system could be better and it's not as good a backpack as a great back pack.