This is an invalid argument in my opinion for one simple fact and reason, ...oh wait I have 3 great reasons!
#1: Other classes get strong utility as well.
-Dragoon's crit buff is absolutely insane (15%!!!?!!?!) and increases piercing damage
-BLM gets Apochiasisis and is outright the king of AoE with no rival
-Monk gets enhanced mantra, removes buffs from enemies and reduces their intelligence and resistance by 10%
-SMN gets eye for an eye and supervirus
-Ninjas have smokescreen, trick attack (Oh and let me make clear what this ability does just to rub it in really well: Increases target's damage taken by 10% for 10s on a 60s cd timer.) And Goad (TP REGAIN) and Shadewalker
Your argument that the other DPS clasess do not bring strong utility is false at the highest level, it does not hold merit.
#2: THE DAMAGE REDUCTION IS BUILT INTO THEIR KITS.
BUILT. INTO. THEIR. KITS. AS. IS.
Bard gets a 15% damage reduction to ALL OF THEIR ABILITIES.
Mach absolutely loses their turret's dps contribution (80 potency spammed) and also invalidates their level 58 hypercharge ability! THIS IS NOTICABLE IN PRACTICE FOR CERTAIN.
It is not even close to the fact that these skills go unchecked. The DPS loss is significant and adds up QUICKLY when the songs are used. 10 seconds of the songs even is 10*DPS_LOSS. And mana is built into their kits and is lost as well, these skills do NOT come without significant tethers and DPS losses. They are already built to punish people significantly who activate the effects. These are NOT "pop and forget" abilities, they come at a premium. Also even if someone is naïve enough to leave them on they will run OOM quickly and be unable to provide this utility later.
#3: DPS CHECKS IN RAIDS.
BARDS AND MACHINISTS TAKE UP A DPS SLOT. Go try Ravana EX with multiple bards and machinists, you simply can not do it.
Then try it with a full team of monks or dragoons and see it be the easiest clear you've ever had (I speak from personal experience).
At present taking more than ONE OF a Bard or Machinist is a liability at DPS checks. Either as a healer (Me) you have to DPS to make up for the lost damage or you will not pass.
"But the numbers in dps calculators say it isn't that much lower", HONEY IN PRACTICE IT IS THAT MUCH LOWER. Whether that be because of cast times or whatever else it is just flat out that much lower at present.
This isn't acceptable for two classes to be so below the rest in the same role that it is almost abhorred to take even a bard and a machinist (one of each). It is an EXCLUSIVE OR Bard or machinist if not take neither at present.
However because of healers mana demands they are granted a spot begrudgingly in statics/ party finder groups.
Closing statement ------------
So why do we have this mentality that Bards and Machinists MUST have at least 20% lower DPS... BEFORE EVEN USING THEIR MANASONG/LOSING THEIR CRITICAL TURRET? (So even lower?!?!?!?!)
They have also been forced to eat the massive "nerf" of casting times that now they cannot move much at all compared to before with WM and Gauss. In other words their playstyle has been crippled ontop of their lower damage.
So I strongly feel that this argument that bards/machinists should have lower DPS at any given moment than other DPS classes is an invalid statement that should be made obsolete.