So, out of curiosity, I went and tested the nerf. Since damage values on ruin aren't changed, this was actually simple. I timed how long it took to cast thunder and the effect's duration. I then cast thunder on a target and spammed ruin until thunder ran out. On the second target, I simply spammed ruin for the same duration (yes, including the time it takes to cast thunder). I did this 100 times to make sure I had a decently sized test pool. Here is a screenshot of the typical difference of damage. The target on the right is the thunder + ruin target. Target on the left is ruin only for the same duration:

So, yeah. So much for SPAM RUIN MOAR. Keep in mind that's 20.42 seconds (give or take a half a second of human reaction time) of damage. Imagine a Titan HM, or coil bosses, and the amount of damage loss is staggering.