You have pretty much answered your own question.
G + Y have independent power sources and a failure in one will still give you control of the other flight surface. If you MEL the ELAC controlling the independent sources (G+Y) and you happen to lose the blue system you lose the flight surfaces. It is probably more important for aileron control. The elevators have SEC's as back up as well.
Have a look and the hierarchy and failures of each system and which computer takes over the control surface next.