If Yamato indeed never joins, odds are Oda was only baiting the readers, who at the end of the day are customers. One Piece isn't independent art, it's a multimillionaire empire; whoever comes here expecting great writing is deluding themselves because it's a product first and foremost, and considering Yamato's popularity and the publicity coming from the debates around him I believe Oda did a good job in this regard (as puerile as it sounds).