Hello Bill, The jacks don't "have to" ground to the enclosure, but the enclosure needs to be grounded to the circuit so with plastic jacks you'd need to come up with another scheme to ground the enclosure to the circuit.. The plastic jacks' grounds and the enclosure ground would be seperately wired to circuit ground where with the metal jacks, they are hopefully solidly connected to the enclosure (starwasher) so the jack ground wire will also ground the enclosure.
With metal jacks you'll often see only one of the jacks ground connected to circuit ground relying on the enclosure to complete the ground connection to the other jack.
dave