I don't think it'd be possible to keep hate on endgame content without Shield Oath and Defiance. Scholar wouldn't even be a healing option, Summoner wouldn't be a DPS option. Conjurer wouldn't have Regen or Divine Seal to enhance healing.

THM would probably be the class least affected, but the lack of Manawall for survivability and Flare for utility would gimp them.

In short, it's just not viable in a group setting to only use the base classes. The cross class skills won't make up for the lack of job abilities in most cases.