What uses of diplomacy wouldn't this cover?

[claudekennilol]

An empiricist investigators uses int Diplomacy checks made to gather information.

Student of Philosophy (social trait) lets you use your int to Diplomacy checks to persuade others.

With those two above, is there anything that Diplomacy would still use cha for?

[DominusMegadeus]

Depends if "persuade others" means 'Make Request', 'Influence Attitude' or both.

Whether or not you can use Int to make requests also determines if you can use it for the Diplomacy portion of 'Suggest Course of Action', as well.

You can find all 4 uses listed here.

[Claxon]

I agree with Dominus. It's kind of a grey area because "persuade others" isn't defined.