Intersex people and those going through transition to alternate gender identities (other than MtF and FtM) have no current way to express their gender properly on their drivers' licenses, identification cards, and passports. They are forced to identify along the binary, which can make them uncomfortable.
Creating a third indicator for these people would bring in revenue from people updating these documents to fit their profile more accurately. X is currently being utilized in Australia, this could also be used in the US.