Going off topic here, but I was under the understanding that some +song instruments increased potency rather than duration. Are you telling me they all increase in duration? For instance, I thought Coronet increased in potency, but kept the same duration as just any old wind instrument.
Any "old school" (as in, before BRD was the main puller at lolibri-only camps) BRD can say with certainty that +song effects increase duration as well as potency.
Way back when melee didn't kill an exp mob in 10~ seconds (AKA The Dark Times), even a pulling BRD would generally find themselves with one mob being fought, one slept at camp waiting, and still plenty of time to sing a full round of buffs for both melee and mages. At the time, I personally would /whm to those parties so ballads were my preferred active-on-me songs, leading to 2 melee songs > run back to mages quickly > 2 ballads rotations.
The extra 24 seconds from your +2 song instruments becomes a nuisance fairly quickly in this situation when you consistently end up with 1 melee song and 1 ballad on yourself instead of the 2 ballads that you just sang.
Or you could always verify it for yourself with ~5 minutes of testing and a stopwatch :P