while i think filling up friendly matches with npc's would be a great idea (and square should really think about adding that) i get why it isn't possible. any kind of game for ranking is obviously out when you sign up with more then one person and the way Mahjong works would make it incredibly easy to boost a single person to victory if two or more were working together. but as for friendly matches ? damn let them fill up with npc's, and if two or three friends think they should team up to bully the npc and that's there way of having fun ? so what, the npc won't complain, and with zero rewards besides fun so what?
edit: another thing they could potentially add would be some kind of "team mahjong" allowing you to sign up with 2 people and get paired up with 2 other people playing the games in teams.