Easy answer: not everyone's gcd timer is the same. NIN has a speed boost, MNK has a different speed boost that ramps up, different gearsets have different skillspeed/spellspeed, also latency can delay your next skill when ogcd skills are used.

Why make the metric more vague (by saying potency) when all that does is make it more inaccurate? Damage per second is the exact metric needed and is as precise as such a metric can be.