I understand your point. This is easy to factor when you just look at two DPS side by side.
The most common party composition is:
2 Melee
1 Caster
1 Bard
1 Pal
1 War
1 SCH
1 WHM
I'll throw some I guess average #s out there for turn 13. (average - not highest)
WHM DPS is usually pretty much a non-factor in FCOB - but everywhere else it also matters.
SCH - 185 DPS (Their WD is scaling properly)
OT - 230 (Their WD is scaling properly - same as Melee DPS)
MT - 175 (Their WD is scaling properly - same as Melee DPS)
BLM - 435 (Their WD is scaling properly)
MNK - 470 (Their WD is scaling properly)
NIN - 460 (Their WD is scaling properly)
DRG - 450 (Their WD is scaling properly)
BRD - 415 (Their WD is 10% less than melee and tank every patch)
Ok, that's 6 spots in the party that's WD is scaling normally. I'm not sure how to calculate how many percent higher a casters / sch weapon is compared to a bard, but I'd also say 5 or 6% after conversion of magic damage vs physical damage.
Now when you compound this WD percentage x 6 (or 7) other jobs that weapon scales at 10% higher than a bard. It basically compounds to subtract what a bard brings to the table for every fight.
That's 7 spots in the party that's WD is scaling at a higher percentage rate than a bard.
It's easy to say ok Bard, you're going to do 10% (subjectively) less damage than the DRG when the two of you compose the entire party.
But in 8 man raids, 4 man dungeons, this is not the case.
So 1 vs 1 a DRG would do 10% higher.
But when you compound the 6 or 7 other jobs that weapons scale higher, this is what diminishes the bards return every patch.
This is why I feel a Bards WD should be x number WD lower than melee vs 10% lower than melee.
It's not a 1 vs 1 completion.
This is why I feel bard weapon should scale at 5WD less than melee. The other DPS is always going to do more damage.