I think the dealerships have done 90%+ of body repairs. While generic body shops may be capable, they also may not have a clear understanding of some of the subtleties of what makes a Smart a rigid, complete safety cage. The Smart dealerships have a much deeper understanding.
As it is, most body shops may not have proper numbers on estimated times and material costs to generate a proper estimate. I bumped into this issue when I was getting a custom-painted panel replaced.