Seems there are diminishing returns on legendary drop rates.

http://www.mmo-champion.com/threads/...rs-Spreadsheet

If true its just not going to be very likely to get best in slot legendaries for multiple specs.

While its true most people don't need legendaries to do current content and its mostly just for mythic raiders, but multiboxers need the extra dps and utility as we generally need to outgear the content greatly.