Like everything a SMN does, it's situational. If I just need to do something in between cooldowns/refreshing DoTs, I choose Ruin I (or even a non-Dreadwyrm Trance Ruin III depending on my MP situation). If I need to move out of the way of an aoe, or want to throw in some OGCD skills, use II. Another reason not to spam II is the blind debuff has diminishing returns; after a point it's just unnecessarily using the extra MP for no real benefit. As for the run in, run out for Miasma II, unless the party is on the strugglebus DPS wise, I don't really use Miasma II all that much...