Actually, the master rank probably gets you more storage (i don't know, i haven't gotten it) and the secret rank is not useless. after you get it, you can take special wonder mail missions to get 7 "treasures" in dungeons with exclusive legendaries (Articuno, Mew, Kyogre, Groudon, Rayquaza, Heatran, and Giratina). to get the wonder mail, go to gamefaqs.com. they have all of the special legendary codes. plus, you're almost guarenteed to get these "guardian" legendaries on the first time through! i got over half on my first time through each dungeon. once you get to the end of one of these dungeons, you'll know what i mean by "guardian."