Why are skill speed & spell speed separate stats?

[Nicobo]

These are the only two secondary stats (i'm not counting parry here) that don't effect all abilities. Why is that?...

I think that is just bcoz the abilities are base on TP (Weapon Skill) or MP (Magic Spell).
For example when a WAR cannot use WS, he still can cast Flash.

Skill Speed reduces the GCD and Spell Speed reduces the casting time.
Flash, sword/shield oath are instant cast spells, I think Spell Speed also no effect on them.