Just to add a few pennies to the debate, in general dry spots are caused by one of two things: lack of pressure, or excessive pressure.
A small dry spot is probably caused by excess pressure, and you are likely to find pain in the horse in the area of such a dry spot. Larger dry spots are more commonly caused by lack of pressure. In this case, you might expect to find pain in other areas...that is, in the areas of the horse's back that are getting excess pressure since the dry spots are getting none.
Even sweat patterns are only a small part of the whole picture of good saddle fit, but they are a basic requirement.