Making Ruin into a auto attack would, from a design standpoint, be a terrible idea. The amount of downtime that would be added to the SMN's rotation would make the class boring and lackluster to play. Look at every DPS job in the game, Dragoon, Monk, Black Mage, Bard, and Summoner: unless the melee are out of TP (which should rarely if ever happen) or the summoner is out of mana (again, should never happen) all these classes always have something to press, which makes the game more engaging and interesting. If you took away ruin as a cast and turned it into the auto attack it would make the SMN just pure unfun to play. And considering the amount of rage I see at SMN's design in the first place, that's something that would not be healthy.