The only real reason why Skill Speed is horrible for NIN is cuz of Ninjutsu. If your global CD is back before you can finish a spell, then you are losing global CD time. The faster your global CD, the more is wasted. The reason why Huton is amazing despite that is cuz:

1) It's super powerful. 15% is amazing, it would still be great even if it didn't include auto-attacks. You could stack all out Skill Speed and probably won't even reach 10% of what Huton would give you in CD reduction alone. Note that it takes much more to reduce your CD by an absolute amount the lower your CD is but the benefit are also much greater (e.g. it takes much more Skill Speed to reduce 0.1 s from a 2.0 s CD than it does from a 2.5 s CD but the damage % increase is also much greater).

2) It's cheap. Giving up a Raiton for a 70s buff this powerful is a no-brainer.