I used to tour on a Bianchi Volpe. When I first got it, the bike was fine for touring. Later after I put on 30 lbs, the bike began to shimmy. Nothing I did, and I tried lots of things, fixed the shimmy. For me, shimmy meant that the bike flexed left to right and front to back as I rode it. Yes, you could ride it that way. Yes, I found it disturbing to ride. A 30 lb weight gain should not have made that kind of difference in the ride quality.
The Volpe frame was made from Reynolds 520 alloy, which if memory served me right, is designed for light applications. So I am left to believe that touring bikes should never be made out of 520/525 alloy. 520's and LHT's are made from generic chromoly tubing, so cannot comment on the properties of their tube sets. That is why whenever the question comes up about what kind of bike should be purchased, I steer them to a Jamis Aurora Elite with its stiffer Reynolds 631 frame. The Elite looks good on paper, but I don't know anyone that has actually ridden one.
I think the real question becomes why can some people tour with a $1000 bicycle just fine, and why do these same bicycles shimmy for other riders.
My belief is that things are just more complicated. If you are 5'-3" and weight 135 lbs you can ride anything. Once you get bigger, or heavier, or have other issues like a noisy upper body, it just gets more difficult to have a shimmy free ride. The bike components and wheels of a $1000 bike are certainly up to the task--I don't know why frames are the problem. The same components and wheels might fail on an epic around the world trip, but they will get you coast to coast in the US of A.
Could it be that the introduction of aluminum frames caused the design of steel frames to be perverted in an effort to take weight out? Could it be that it is just too expensive to make steel stiff enough for a touring bike? Do you need to add super exotic stuff? Why can you just make the tubing diameter bigger? Your more of a materials scientist that I will ever be, whitierrider, what is your take?
All I know is that you hear a lot about cheaper bikes shimmying. I know a 520 owner, and his bike shimmys. I have read about LHT that shimmy. I have even heard about Bruce Gordon's shimmying, and that is not a cheap bike.
I know that I overreacted to my Volpe's shimmy by buying one of those really expensive custom bikes that mrpincher was poo-pooing. My $5000 Waterford is stiff. I have also deliberately flexed it, and she quickly dampens the flex. I don't know what Waterford did to my frame that cannot be done for little or no added expense to a pedestrian steel frame. Now for $5000 I got to do some other really cool stuff like non-dished tandem wheels and a drag brake, but I only really needed a better frame.
If my Volpe's frame had been stiffer, I would have kept riding it. The fit was good, the components were good enough, and the bike was great to ride unloaded.