Basically, MME was delayed, and as a result, it delayed useful implementation of the rarity system. With that book now out, there are some legitimate rare items, and a solid number of common items. They could have made a bunch of items common early on, but without rares, it would have just been a case of "things to deny your players", and to a lesser extent "here is some stuff you can sell for more money, not that there is anything you can buy with it anyway". Now that there are at least enough items to have a clear indication of what types of items warrant what level of rarity, it enables a DM to be able to adjudicate it themelves (since, most of the books with those rarities won't get updated even if they are errata'd). In the long run, it will be nice to get the compendium/character builder updated with that stuff, but having standards that can be assigned independently, vs. just assigning the rarities, at least gives people an explanation, and the tools to decide for themselves (maybe they don't feel iron armbands of power can be common in their game).