I would have thought that way too until I saw the way that her rack was swaying. We were using a 60/40 split and her load was significantly less than mine (I was carrying all the heavy cooking gear). Her rear load consisted of a sleeping bag and pad, rain gear, clothing and a few personal items. None of it was heavy, just bulky but the rack swayed through quite an arch. She probably wouldn't have noticed it nor would most riders. But to an outside observer, it was quite clear. We replaced the rack in Lewiston with, I think, a Topeak which was better but still not as stiff as my Tubus.

Personally, I believe that a lot of death wobble problems (her bike developed one at around 30 mph on the downhill side of Lolo Pass) in touring bikes is do to the lack of stiffness of rear racks rather than a lack of stiffness in the frame. My old Miyata had a Blackburn style rack and it would wobble at around 50 mph. My Cannondale doesn't wobble at any speed that I've been stupid enough to take it to. I could also never stand and pedal my Miyata with a load...it just wandered all over the county. The Cannondale I can muscle up anything out of the saddle and can throw it from side to side like a race bike without it problems. The Cannondale's frame is stiffer than the Miyata but so is the rack.