Alex,
I haven't seen all of the email on this subject, so forgive me if this was all ready mentioned or if it "goes with out saying" but one faily common case would be when a multi-homed customer prefers on provider as primary and the other/s as a backup. The primary provider would prefer customer announced routes over that of any peers, while the backup providers would set there preferences to normal customers at highest, then peers, and then the customer designated backup at lowest priority.