All depends on the content, for small NMs, lighter exp party, questing etc a THM could easily heal and PGL and MRD could easily tank, but this depends on their abilites available(for example Cure spells come from CNJ but once unlocked you can use them while on any other class including DD or tanks).

Before the job system is here at the moment, it depends how versitile you make your character for how much ease you can get into groups for encounters.
For example a DD with many classes leveled to a certain point (lets say all other dd jobs to 20) will be stronger and more resourceful then someone who only leveled one job at all.

Character progression may sound very daunting like this but for myself I think this is very fun and exciting way to encourage trying out and leveling multiple classes as I feel my character becomming stronger with each ability I unlock.