You've always been able to use cross-class abilities. Just because you equip a job doesn't mean you will lose this option, it will just be limited a bit.

As a PLD, you'll still be able to make great use of any other skills you've acquired (those form MRD and PGL being the most helpful to a tanking class) - but there will be some offensive abilities and powerhouse weaponskills that you'll lose access to as a result, regardless of their classes of origin.