Why is the use of morphological traits to distinguish species considered to be a disadvantage?

The use of morphological traits to distinguish species is considered to be a disadvantage.In some instances,slight variations within a species,such as size differences between the sexes,may be misinterpreted as evidence of the presence of two separate species.Conversely,the lack of diagnostic differences could cause one to conclude that two living organisms or two fossils are the same species when they are not.