Quote from Narvuntien »you need to be able to build interactive and aggressive decks from mostly uncommons with a few key rares.
Quote from DirkGently »WotC needs new product to sell, and eternal formats don't make for big sales. I think that's also likely why they're tying the precons to the sets, so that you're forced to buy the set to use the mechanics to the fullest.
Not sure I see the connection between companions and Eminence. None of them do anything until they're cast. They might be an attempt to entice commander players, but I think more likely it's just a way to use the fun gimmick of commander in non-commander formats. I think the most defining quality of commander is being casual, so fans of commander aren't likely too keen on getting into a competitive format just because it imitates one mechanic.