Sometimes you just have to use Ruin II to keep moving, particularly when Bio II or Miasma II is already up or you're too far away. That's better than using no GCD at all. Examples are healer Black Holes in ye olde v4s, or running away from Ghost Train for preys/Head On, etc. Basically, if you're gonna be moving for an entire GCD, Ruin II is much better, at least if you don't wanna burn Swiftcast on DPSing :P

As KDSilver said too, if your weave is ED+Shadow Flare, Ruin II is still a viable option, though definitely not preferred.

For weaving a single healing oGCD or using Aetherflow, I don't think Ruin II is a gain though. I'd rather clip if there's no other option.