I think it would be good to evaluate whether the secret skills are worth getting, so I'm going to look at the description and write what I think of the skill.
(*edit) Color Coded it: green for defensive buff, blue for offensive buff, dark red for aoe, orange for debuff, and red for single target attacks(I'm counting shaman as single because it doesn't say a range or shape for an aoe, though I don't know)
Warrior: The skill is a defensive buff for parry and block.
Frankly, to me it looks useless. I mean, I guess if you had a 2h weapon on the parry would be good, but axe wielders tend to be dps warriors, and block caps out at 50% so the buff probably won't help much as warrior tanks usually get high enough stats to cap it unbuffed.
Knight: DoT which reduces move speed -20% for 12 seconds. This skill actually looks useful, though it depends highly on how much the DoT actually does, reducing move speed in addition to the move speed buff knight already has can make it easy use hit and run tactics, plus, it's a 3rd DoT skill, which makes it an effective counter to an enemy knight.
Templar: Increases attack speed and double hit rate, but reduces move speed 40%. I'd say this one is useless. Though the 15% double hit rate may look appealing to a templar, the 40% move speed penalty makes it a no go, and since templars use magic, attack speed isn't good at all.
Dragon Knight: Essentially decreases Luck(acc, mcrit rate, crit rate) with an aoe. I'd say this one is actually pretty good, though it depends how much. If it's a decent amount it would be an awesome attack in a territory war, since it not only affects dps, but healers as well(less crit heals). Not to mention how useful it would be tanking to decrease enemy offensive stats.
Thief: An attack skill which decreases crit rate and magic crit rate. I'd say the usefullness of this depends on the actual attack's power, dragon knight's crit reduction is only good because it's an aoe, single target debuffs like this don't usually have much use.
Martial Artist: Increases move speed and decreases malice. This is a good skill, martial artists can get a lot of malice due to that stacking attack speed and power buff, and the move speed bonus is just awesome.
Blade Dancer: Removes all debuffs, and increases attack spd/acc. That's pretty **** good. debuff removal skills are just awesome, and for a melee dps, you can never have too many attack speed buffs.
Samurai: 15 ft aoe. Simple skill, good skill.
Hunter: Healing over time skill. Eh, not that good, unless it heals a lot. If it heals a lot, you could probably use try to use it to be cheap by running away, stopping/slowing enemies while you heal, but even then, it probably wouldn't help much in most situations.
Engineer: 40% chance knockdown aoe IN MECHA FORM! **** YEAH! There is no reason to NOT want this skill.
Ranger: 15% elemental resistsances and def. 15%. THAT is good! If you are human, it essentially means 25% elemental resists just for being ranger. Not to mention, it's 1 minute long! Combined with ranger's other defensive stuff, this is a great skill to have.
Cleric: DoT that reduces elemental (*edit* and physical) resistance(5 stacking, 2-10% resist drop). I'd say this is pretty good. Combined with cleric's holy/nature debuff, you get -20% resistance for your skills, plus it's great for taking down a boss because it helps out the mages a lot.
Bard: Wedge lightning AoE. Not that useful. Bards are healers and occasionally part-dps, but that's usually only good on boss fights, and aoes aren't.
Shaman: Not sure if this is an aoe from the description, but I'd say it isn't useful.
Sage: Reduces EVA and stops movement. Pfft. Like anyone cares about sage.
Magician: 1 min malice reduction eva increasing buff. This is a good skill, mages need malice reduction in dungeons, and eva is always good.
Illusionist: 15 ft Wedge aoe, with 15% knockback. I'd say this is okay. I mean, it's nice to get a decent aoe on a magic class besides mage, but being wedge shaped means you probably need to target someone, which isn't that good, though better than nothing.
Warlock: Dark AoE and DoT, within a 10ft range of caster. Decent, I guess it's another aoe, but it's 10ft around you, like lightning web, the uses are very limited.