Ofcourse business casual is the proper way to dress for field work. However, appraisers often have to work outside in both winter and summer environments so you have to take that into consideration. We have 2-3 feet of snow here in CT. and to walk around the exterior of a home taking measurements, notes, and photos can be messy and hazardous. I came home the other day and my pants, socks, and shoes were soaking wet. In the summer the heat, mud, tall grass, etc. can be the devil to a nice pair of shoes and slacks.
So while I agree some form of business casual should be maintained dressing too well should be avoided as well IMO.