Though at 10% it might as well be 100%, drop rates are a funny thing and you can't make something scarce or hard to obtain with a double digit drop rate. There really are not other ways to do rarity in these games frankly. You can either put things behind multiple raid bosses and even then it seems like a ton of players get them (are there any achievement statistics sites yet?). I mean this really is just one of the few ways to achieve rarity, you either make an item really low drop on easy plentiful mobs, somewhat low drop on really rare hard to kill mobs, or lock it behind some highly overtuned raid that will take months for people to learn. I guess the only other thing would be making the item behind some bracket PVP tournament. There just are not a lot of other options, beside just making it easy to obtain.