All DOM starts its life as welded seam tube. It is drawn over a mandrel to assure its shape is true. This process removes material and makes the wall thinner. It is only stronger than regular tubing if the regular tubing was not made properly. DOM will correct tubing that is somewhat out of round. DOM will dent just as easily and maybe even easier since it is thinner. The racing industry specifies its use for liability reasons. They dont want some guy making roll cages from egg shaped tubing.
If regular tubing is made properly it should be just as strong as DOM. People say and believe all sorts of stuff when it comes to DOM. A tube mill will chuckle when you tell them you want DOM because it is stronger. I know.. I did. I purchase over 3 million feet of tubing a year and I thought I knew what I was talking about until one day I was taking a tour of a tube mill and they showed me and explained to me all the processes involved.
Some say that the DOM process add strength by "working" the steel and creating some uniformity in the "structure" of the steel. This is probably true to a small degree but most tube mills tell you it is so small they cant measure it.
I am actually considering buying a tube mill... Maybe I can sell you some DOM soon.
Greg