As with anything in this game, Sirvente and Dirge are situational. I personally have my group 2 merits invested as 3 Dirge, 1 Sirvente, 1 Nightingale and 1 Troubadour and wouldn't change a thing. I almost never need to reuse Nightingale and Troubadour as soon as their recasts are up, and honestly don't get all that much use out of Sirvente.
Maybe it's just because I have Gjallarhorn but often when I use Dirge it's enough to turn the mob away from the person it was attacking and back onto the tanks, I've had a lot of mages thank me for using it when they pull hate and no complaints yet. One situation I consider Dirge very valuable at is JoL where we have our kraken drks use souleater one at a time. If the drk doesn't pull hate and die by the time their round is over giving them Dirge at the end allows them to go sit out and wait for their next turn while their enmity degrades faster, or if like at our last JoL where we only had one drk attend it allows them to switch to scythe and melee while still keeping their hate down.
Sirvente was a big disappointment to me, it was one of the first things I merited when the new merits came out and our tanks were unimpressed with its results. Like it's been said it's nice for kited situations where it can be a pain to land aoe songs on a tank, for mixed type tank parties, and for when tanks have buffs or hate stripped, but overall I'd consider this the least useful of the four merits.
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