To be honest, Ruin II is a victim of the shared base-class issue that crops up with SCH and SMN. Ruin II is specifically an ARC ability, and it's one that SMN makes use of heavily in its rotation, meaning that it's very useful there (SMN has a lot of oGCDs to weave, and can proc Ruin IV as an upgrade for it). As an ARC ability, however, it's also shared with SCH, which has better direct damage options and better instant-cast options than Ruin II, which means it's nowhere near as useful for SCH. Basically the only time you should be using it is right before a transition phase if you don't have time to cast Broil II. Source.