Reason # 3
Insects and Disease
The large stubs of a topped tree have a difficult time forming callus. The terminal location of these cuts, as well as their large diameter, prevent the tree's chemically based natural defense system from doing its job. The stubs are highly vulnerable to insect invasion and the spores of decay fungi. If decay is already present in the limb, the opening will speed the spread of the disease.
(Courtesy of the Arbor Day Foundation)