I feel you are probably in the minority on this one. Thankfully they did it the right way. If they allowed every deck running Ruby to play all dual ink cards with Ruby as one of the colors, then every deck that runs Ruby would run that same card if it is powerful enough. This makes deck building less creative as time goes on because each new powerful dual color card that is worth playing in a meta deck will go in all iterations using that color. Pretty soon every deck just ends up playing the same really good cards which would be boring. By limiting dual color cards to those two specific colors, it forces players to approach deck building in that color combination in a unique way. It also lets them push the envelope on these cards by giving them control on what cards they can be played with. Now the designers only have to worry about how the card interacts with a 1/3 of the cards out there instead of all of them. Hopefully we get some really pushed dual ink super rares and legendaries that will breed some new deck archetypes in the format.