I can't think of any situation where you must use yo/tú/él/ella/usted/etc. but I know you do need me/te/le/etc. on a variety of occasions (e.g. reflexives, verbs like doler and gustar, when an action is done to a person/object)
Balance is imperative; without it, total collapse and destruction is imminent.
"...But I shall forgive you. There are some things in life that cannot be achieved." ~Wise man NOT CHANGING THIS SIG UNTIL SUBMACHINE: THE FARMCOMES OUT, which is never :/