TB, your're right we don't feed anything that is 100% DM but you have to use DM when computing the amount of say protein in a ration. Every ingredient has a different level of moisture to it. How would you ever know how many pounds of protein you are feeding if you didn't calculate the DM? You can't use percentages to build rations unless you incorporate the DM factor into those calculations. Besides I don't really worry about the moisture content of the final ration unless it is getting under 60% DM. A cow will eat 60 pounds of 60% DM feed the same as she would eat 42 pounds of 85% DM feed. The cow is eating 36 pounds of DM in either scenario. She is just consuming 18 pounds extra of water which would equate to her drinking fewer pounds of water from the pond, tank, dam. Dilution does not matter because a cow will eat the same amount of DM regardless of the moisture level, assuming the moisture level is not excessively high. In fact in warmer weather a cow will have a higher DMI on higher moisture feed.
So my preference is to calculate in DM to balance out rations and when you determine you need 17 pounds DM of corn silage then use the moisture content to determine how many as fed pounds you need. You can calculate as fed too but most recommendations are based on DM and you will be doing extra calculator work to get the same results. Just my opinion. Have I ever mentioned that I love to debate?