Sup Snipers

You get a skill that lets you wear equipment 10 levels below their requirement, and then the Cygnus skill for your respective job, that gains a point every ten levels with a quest. It's startable at level 70, and you have to hunt Desert Rabbits for some item that drops at such a ridiculously low rate, you're probably better off spending the time leveling to 120 instead of getting a skill that will help you level to 120.

For example, an Archer would get the Wind Archer's Wind Piercing skill when they complete the quest. Then at level 90, every ten levels after that, they would get another quest to level the skill up, I believe, or so the skill description on the extractions say. So I would get it to level 2 at 107 if I manage to complete it at 97. Ironically, with this new update to Blizzard, maxed (level 5) Wind Piercing only does 7% more DPS than Blizzard/Eruption, and the earliest you can get maxed Wind Piercing is 140?, but it's a super sexy skill to have at any level. They may have changed the requirements since the official release, since according to Spadow's blog, both skills are achieved at 70. I'll probably never personally find out as I don't really want to continue with this quest.

In terms of being worthwhile, I think Soul Driver, Vampire, and Shark Wave are the only ones.
Soul Driver just has a lot of range, which alone can give it utility, but when it's maxed, it hits five targets for 250%x4, which Heros can have trouble reaching with just Brave Slash, and Paladins with just ACB. Dark Knights with Fury and Roar have less of this issue since the hit box for those two is pretty big compared to Brave Slash and ACB.
If Shadower is strong enough, Vampire could reduce potion costs, but it only does 90%x4, and only 2% of that, or 7.2% of your damage range is healed to you. Drain is more worthwhile than Vampire, even while hitting 3 targets, and Avenger heavily out-DPSes it with speed.
Shark Wave just has such a huge range, both Vipers and Corsairs could probably find use out of it, whether it be for killing things at a distance or luring.
Flame Curtain doesn't last anywhere near long enough or do very significant damage. Ice/Lightning and Bishops might find some use out of Flame Curtain if they're willing to lay it down every 10~15s, but I don't see it being to great for Fire-Poison's.
Inferno and Pierce easily out-DPS Wind Piercing, so... yeah...

I do believe 120+ characters can do the skill quests as soon as they finish the precious one and not every 10 levels. Or basically anyone post 110?