Why are skill speed & spell speed separate stats?

[Shurrikhan]

Perhaps to keep in mind for future content (perhaps NIN even, depending if Ninjutsu has a cast time) for melee/caster hybrids.
PLD is another example - since PLD doesn't have any spell speed gear (minus materia melds), it means that casting spells like Stoneskin will be at the default 2.5secs recast.

To be honest I feel like those two examples are good reasons to make Speed affect both types. As it stands it's a near worthless stat except in reaching certain plateaus (and still slightly inferior even then). It affects only GCD direct damage - no auto, no oGCD, and no DoT - while Crit and Det both affect all four damage types. If it won't be balanced in a more standard way, it might at least quicken song and Stoneskin casts...