Many have said, you don't have to run as a job. The THM does not nuke badly, the BLM just does it better and he should because it is all he can do. If you want to be a THM/CNJ, by all means do so. I know I wouldn't turn you away from a party because you like more flexibility in a fight. Nothing is wrong with an archer staying an archer b/c he wants all the damage increasing cross skills. A bard as no access to any melee cross skills, so if you DPS is good who cares? Jobs are an optional specializations, you equip one knowing you are going to be the created role. This doesn't mean classes are not just fine in groups.

Jobs have unique and powerful abilities... but they are not required. Some of skill combo are only able to be done as classes. For example, Sanguine Rite + Scared Prism can only be done as a THM. Sanguine Rite as a THM does an awesome damage reduction and with Scared Prism you get the whole party with it. Only the THM can do that, not BLM. CNJ can have Resonance, that is extremely useful as a healer... WHM can't do that though. You see my point. Classes still have value and unique skill combo only they can do.

Play the way you want.