I only do that for longer trips where I'll be in one place for multiple days. Generally I'll get to a hotel, leave the BB in the room and just use the Ristretto/MB/LCB walking around town (laptop + notepad + camera). If I'm going to be hitting multiple cities in one trip, I'll usually just put the laptop in a cache in the front compartment with the other doodads in 3DOCs or a snake charmer etc and save the extra bulk of having another bag.