They pull the -5s CD with most 120s-relevant skills eventually to protect against drift.

Though now that you mention it, I feel the game as a whole might benefit if they changed SkS (and SpS!) to work like this:

* SkS now affects the cooldown of abilities. All of them. All your skills.
* SpS affects the cast-speed of spells. Spells only.
* Neither affects your GCD, which is locked at 2,5s barring any job-specific modifiers to it.

Meaning SkS would give you some drift-resistance to how your skills line up, SpS gives you improved mobility (assuming re-evaluations of the scaling of both given the functional changes, of course!) as a caster. Neither can affect that you use 1 GCD-ability every 2,5s, or as a Monk every 2,0s.

The big upside of this would be that not only would SkS in particular have a real use outside of edge cases, it'd also bake "drift resistance" into a secondary stat. Which could be optimal or not based on a lot of factors, and most importantly is a "soft stat" (it depends on your connection and the weather on that day and whether jupiter and libra align among other things).