Why is Nocturne useless? If someone has enough rubies
For first, indeed it's because of the lack of rubies. But there are more reasons:
- it eats 2 tempo
- if taken with Terror of the Night rubies it would require another Crescendo cast which requires 1 more tempo and takes 1 gcd.
So if only to recover the tempo you'll need to put another cast of Disharmony to your rotation which wastes another gcd. This effectively means you trade ~1 full cast of Dancing Blade for Nocturne. I've made some tests of rotations with and without Nocturne on Dummy and at least for me personally the one without Nocturne produces 10% more DPS.
Whether your group will be able to compensate that loss of 10% of your own dps or not is an open question, because at best you can keep Nocturne up 50% of time (in reality it probably be closer to 30%) and it doesn't boost channeling spells and dots which your teammates might be casting quite often.
So, yeah, spending 7 rubies for that thing is kinda waste.