[…] a principle that we may state in the most general terms as (5), interpreted as imposing a choice of PRO over an overt pronoun where possible: (5) Avoid Pronoun. - Chomsky (1993), a pag.65
The Avoid Pronoun principle operates uniformly, permitting empty subjects where the element AGR is sufficiently "strong" to allow recoverability. - Chomsky (1993), a pag.276