I do agree with the opinion that three Marios is too much. My main reason for not wanting Young Link back is because I don't want a third Link, so I'd be quite the hypocrite to want three Marios.
At the same time.....there's not much competition left for the Mario series. Rosalina and Bowser Jr were the most important characters in the main series after the ones that are already Smash veterans (excluding Toad, who's doomed to Neutral B hell).
I'd rather have a unique character, but none of the remaining unique Mario characters are that good or important. Paper Mario's one of the best choices left.
....And before anyone says "But he would be unique!", I meant in regard to appearance. Paper Mario would obviously fight much differently than regular Mario and Doc, but he's still a short little mustached man in red.