If you don't mind having long timers on Nightingale/Troubadour you can also go for Dirge. Dirge isn't very commonly desired, but it sure can be helpful in certain situations as it provides -10 + -3 per extra point invested.
Good uses for Dirge:
Chainspell Stunners
BLMs
certain DDs using 2-hour
Basically, whenever someone in your party is going to do something that will likely cause them to die, putting Dirge on them will keep them alive a little longer. Granted, the song is less effective proportional to the spikedness of the action--for example, Eagle Eye shot is only going to lose 10 enmity, but the effect is enormous when applied to Hundred Fists. The song may keep the monk alive long enough to actually finish the 2 hour effect. The maximum effect of Dirge is 10 + 3*4 = -22 Enmity which is
huge. If you're only using Nightingale/Troubadour for zerg buffing or in conjunction with your two-hour (sing the first set of buffs normally and then sing the second set with N/T on just before your Soul Voice timer expires for maximum duration), then you don't need the lowered recast time and you can afford to drop all 5 points into Dirge. Monks and Black Mages will thank you for helping to keep their buffers intact.
Make sure you know when your recipients are going to use their two hours in advance and cast Dirge at the beginning of the fight before they have any enmity at all for maximum effect.
Just remember that Dirge doesn't take away existing enmity or make variable enmity go away faster--it reduces the enmity of each action taken
after it goes into effect, so you must use it in a proactive manner for it to be worthwhile.
As for Sirvente, A NIN or /NIN tank will always want March x2 for Utsusemi recasts over anything else. Most NINs won't trade 9% Haste (lil' March) for a reduction in enmity loss. Paladins usually don't need this at all and the shortened recast on Flash or the additional accuracy/attack is probably going to benefit them more than the enmity-loss reduction. I can, however, see Sirvente as being useful for unconventional tanks like RDM/NIN. The song is mostly meh, but if you have extra merits laying around (you won't) go ahead and get it if you're not going for Night/Troub x 5.
Also Singing and Wind merits are stronger than I may have previously let on--They're just not usually necessary for buff sticking.
Let's examine them numerically for a few songs:
Minuet IV caps at +56 Attack and a Bard reaches this cap when he has a combined skill of 462. Bard skills cap (naturally) at 225 for a total base skill of 450 which means you only need +12 to cap out and you can get that just by getting yourself a pair of
Choral Cuffs +1 from Limbus and your
Bard's Roundlet from Dynamis, so you definitely don't need merits for Minuet. There are also cheaper sets: Minstrel's Coat (3), Bard's Roundlet (5), Trumpet Ring x2 (4) is the cheapest but requires Dynamis and NM camping.
By contrast, the last March tick (11%) requires 486 skill which means that you need +36 total skill to get it. To get that with gear you need to have (for example)
Choral Cuffs +1 (10 Singing) ~227k
Choral Cannions +1 (8 Wind) ~227k
Wind Torque (7 Wind) ~400k
Musical Earring (5 Wind) ~750k
Bard's Roundlet (5 Singing) free!
Trumpet Ring (2 Wind) ~10k
This is a fairly affordable March build that doesn't require any merit skill. If you can't afford the time/expense for this setup, then you can get there with less and use your merits to bridge the gap. This gear set costs about 1.6 million gil on Asura.
Finally, for Madrigal, the cap is very high,
508, so if you're trying to cap Madrigal, you may have to put some merits in to get to +58 unless you have a good bit of money and time to spend on gear.
Let's take a more potent gear set:
Choral Cuffs +1 (10 Singing) ~257k
Choral Cannions +1 (8 Wind) ~257k
Marduk's Tiara (7 Singing) ~2.2m
Wind Torque (7 Singing) ~400k
Musical Earring (5 Wind) ~750k
Astute Cape (5 Wind) free!
Oracle's Pigachets (5 Wind)
New ZNM Item free!
Minstrel's Coat (3 Wind) free!
Singing Earring (3 Singing) ~400k
Nereid Ring x 2 (6 Wind) ~260k
The total for this rather expensive gear set is +59. This is the maximum possible +Skill build currently and it utilizes all of your armor slots to maximize the effect of Madrigal. Technically you can ditch one of the Nereid Rings for a Trumpet instead--you don't need both rings HQ. This gear set costs about 4.5 million gil on Asura.
The
Magic Accuracy effect of skill, however, will be quite significant, but I would recommend getting your HQ staves and a CHR/skill equipment build
before meriting. +CHR is quite easy to get, but it probably loses effectiveness at some soft cap--this effect is not documented. At some point you'll probably be better off adding skill and Magic Accuracy over CHR, but I can't tell you where that point is. If you want more Magic Accuracy and have exhausted your gear resources, go for as many as you like, but don't think that you
need skill merits to be a good Bard. Skill merits can be used to replace high-end gear, though.