I had a friend with the backpack accessory for commuting. The nature of our work required us to be on and off the bike repeatedly throughout the day, and to bring stuff with us. Because mounting the backpack stuff is slightly clumsy, it didn't take long until she was carrying it by the shoulder strap, letting the half attached backpack straps dangle.

For shopping, I just latch my panniers onto the shopping cart.

My new favorite bag is the REI Flash 18. It's just big enough for a long day hike, weighs almost nothing, rolls up into a tiny ball and I can wear it all day without any discomfort. I'm trying to get my touring load to fit into two front panniers and the Flash on the rear rack with anything I might need for a quick jaunt off the bike.