Much like how Medics are The Healer Class, Troubadours are The Buff Class. They provide buffs for party members, along with one skill that debuffs enemies. That's it, really. They provide buffs and then they sit around probably tossing out items until the buffs they casted run out. They're not nearly as broken as they are in EO2U, due to the lack of Crusade, but they're still pretty significant sources of amping up your damage dealers' output. They can do that same diminishing return system-bypassing that they could in EO2U, but it's not as big a deal as it was in that game, since EOU punishes you for that less.


Stats

(Full stat table)

Common Passives: HP Up, TP Up
Gathering Skill: Take

Troubadours have HP that's a bit on the low side, high TP, AGI, and LUC, and throughly mediocre STR, TEC, and VIT. Very little of that matters for them besides TP, because buffs are stat independent.


Song Mastery


Required to learn song skills. Increases the speed of song skills.

Warrior Song


Increases all party members' attack for a set amount of turns. Has a 60% speed modifier at all levels.
Warrior Song is the most basic and most important song skill. You're probably gonna end up casting it at the start of every fight. You can immediately see a pattern that'll be reflected in the rest of the Troubadour skills here, though: the actual buff effects scale really terribly with each skill point. In Warrior Song's case, two of its levels buff its effect by a measly 2%, while the rest buff it by 1% or none, because the duration gets bumped up at major levels.
This is the most conventional of the songs and it is generally useful to have for a lot of the game. There's definitely better damage skills out there, but 30% at Lv.10 is pretty good.

Shelter Song


Increases all party members' defense for a set amount of turns. Has a 60% speed modifier at all levels.
Defense buffs aren't as generically useful as attack buffs, but they're still pretty good. EOU has a lot of enemies, from random encounters to FOEs to bosses, that deal enough damage to the point that a 20% reduction to that damage can be the difference between someone barely hanging on, and getting one-shot instead.
This takes a bit to actually start taking effect, but 20% is a pretty good reduction amount against certain scenarios. Not necessarily the greatest skill to take, but it's not exactly worthless either.

Holy Gift


Increases EXP gained from battle while the user is alive.
Effectively 21 skill points for a 25% increase to battle EXP is a terrible investment of skill points.
It's good to have for retire grind cycles, but I'd rest it off once you've gotten to where you want to be.

Skanda Song


Increases all party members' speed for a set amount of turns. Has a 60% speed modifier at all levels.
Speed buffs. Blech. There's one or two random encounters where being slower than a given enemy is a big issue, but on the other hand, Skanda Song has to go before the enemy for you to have any hope that it'll actually change anything.
The edge cases for speed buffs are too few to warrant running this, and more often that not you'll still get outsped regardless.

Insight Melody


Increases all party members' accuracy for a set ammount of turns. Has a 60% speed modifier at all levels.
Accuracy buffs have a use when you have party members that have skills with accuracy penalties, or when you're going up against that one boss that reduces your accuracy.
S3 midboss buster. It's useful because that gimmick is insufferable but beyond that it's best left alone.

Recovery Aria


Increaes the rate at which party members recover from ailments and binds. Has a 60% speed modifier at all levels.
Using a buff slot to make yourself recover from potential disaster a little bit quicker isn't the best use of skill points, in my opinion.
Don't trust ailment recovery. Deal with the situation directly instead or you're probably toast.

Fire Prelude / Ice Prelude / Volt Prelude


Applies a fire/ice/volt element to one party member's attacks. Also increases that party member's damage--this does not count as an attack buff. Has a 60% speed modifier at all levels.
Those of you who remember EO2U probably remember these things as "those skills that let you bypass the dimishing returns system." Well, they're in EOU, and they can still fill that role, but they're ever-so-slightly weaker than they were in EO2U.
This is useful for edging out a little bit more damage on a damage dealer and also setting up for elemental chasers.

Flame Fantasia / Frost Fantasia / Shock Fantasisa


Reduces all party members' vulnerability to fire/ice/volt and increases all enemies' vulnerability to the same element for a set amount of turns. Has a 60% speed modifier at all levels.
Same deal as EO2U: toss these out to buff up elemental damage while also bypassing the diminsihing returns system.
This is a good skill for any party with heavy element usage. The two main ones I can immediately think of are chaser parties and the Story party. However, the party resistance boost is also pretty strong too if you need element coverage.

Echoing Rondo


Places a buff on all party members that heals them when a party member is healed. The caster's TEC is used in determining heal amounts. Has a 60% speed modifier at all levels.
Echoing Rondo is a very situational skill, especially given that Efficiency-boosted Somas and/or Delayed Heal will take care of most of your healing needs.
This is a bit overkill given the general strength of the other healing skills in the game. I guess you could use this to save TP on a Medic?

Healing Lullaby


Restores all party members' HP at the end of each turn for a set amount of turns. Has a 60% speed modifier at all levels.
Doesn't restore nearly enough HP for it to be worth both the turn spent casting it and the buff slot.
Pretty undertuned to deal with EOU's damage trading like most regeneration effects in the game. You can use it as a background extra healing source, but don't make it your mainline healing skill.

Eerie Chorus


Removes buffs from all enemies. Has a 60% speed modifier at all levels.
Level 4 of Eerie Chorus is enough, honestly. You're basically never going to run into an enemy that has two or more buffs at any given time.
Just chuck a Metopon, really.

Peace Ballad


Restores all party members' TP at the end of each turn for a set amount of turns. Has a 60% speed modifier at all levels.
Restores a pitiful amount of TP. Not worth the buff slot.
This lost a lot of value from the original due to massive TP cost spikes. It's still somewhat useful, but not really good like it was in 1.

Taboo Rondo


Increases the duration of all buffs and debuffs on all party members and enemies.
Wanna have your Troubadour make it so they don't have to recast buffs, plus make any debuffs you have stick longer? Taboo Rondo's the skill for you. Just...leave it at level 9.
Upkeep on multiple buffs on a single turn of casting is really good for turn economy, especially when the buffs aren't sourced from the Troubadour.

Frightful Yawl


Reduces the encounter rate for a set amount of steps in the Labyrinth.
Goody, just what I always wanted, a way to cut the amount of EXP my party gains.
Oh, hey, it's a gathering party skill except not on the gathering class!

Restful Tune


Can only be used in the Labyrinth. Restores all party members' HP.
130 base heal out of battle for 3 TP isn't that bad, I suppose. The problem is Troubadours are pretty hungry for skill points--you want every SP you can get in their in-battle buffs.
Wow, the scaling on this skill is awful. The Lv.10 version is incredible, but what the hell's up with the rest?

Barbaric March


Increases all party members' maximum HP for a set amount of turns. Has a 60% speed modifier at all levels.
I don't know why the hell Barbaric March is under HP Up, but whatever. If you want your party to be seriously more beefy, you could do worse than Shelter Song plus Barbaric March. That only leaves one buff slot, which will probably be occupied by Warrior Song, yeah, but still.
HP is one of the two actual reliable forms of survivability alongside raw VIT, so I can't really write this skill off. A 45% boost can totally keep a fragile party member from getting annihilated by a strong party-wide area of effect attack.