and prepositional arguments (PIV) have almost as high an expression rate simply because most verbs have alternative valency frames of lower order (intransitive or monotransitive accusative) that the tagger would have chosen in the absence of a PIV argument.
pIV arguments are strong sense markers, and their absence will sooner lead to false-positive senses of lower valency-order than to pIV-senses without surface pIV.