I mean...the potency difference between Ruin II and Ruin III is 40 with both DoTs up. And if you're not weaving you're gonna be losing GCD's eventually. So it seems to me like they intend for us to use Ruin II/Ruin IV to weave the MANY oGCD's we have, and then use Ruin III as our filler when we have nothing else to do. You could also just straight up use Ruin II as your main spell. It wouldn't be optimal, but it would work at least.
In fact it seems like that's the whole point of having Dreadwyrm Trance do what it does, you Tri-Disaster, pop DWT, and just blow as many oGCDs as you can before dumping Deathflare. Instead of just going straight into Deathflare. That way you can use more Ruin III's over all, but this is all spit-balling speculation.
