Ruin 2, should NEVER be spammed, first in your rotation, you're adding a second 2.5 global cool down to you're next cast. Next it puts a 10 second blind up that is on diminishing returns. Believe it or not, that blind can and willl rescue people in different situations. Our class is amazingly unique and fun to play, with many different things to do during a fight.

For example, I LOVE Eye for an Eye and Virus. I typically open every boss encounter with that, plus my pet boost and pet enkindle due to the sick cool down on it. That being said, I run things with pugs and unless I cast it, its never up. SCH / SMN combo can give 20 seconds per minute of all stats reduced, plus 2 applications of a power debuff on a boss.

As per rotation, the Dots are the most important thing to be up like Xylter said. Each tick is huge, and does damage while everyone else is running around. Save Ruin 2 for when a blind is needed, otherwise use ruin when you absolutely have nothing left to do.