It's all optional really. Optional jobs are there for different playstyles, as well as for people who don't want to just sit on one class all day when doing roulettes.

Also, just in case you were not aware, there's an armoury bonus you get for having one job at a higher level than others, combat wise. Other combat classes will get an EXP boost while leveling up, so they will level up faster than your first class. The biggest hurdle for leveling alt classes is usually 1-50. Plus if you unlock a job, you can go into Palace of the Dead to play it at level 50+ just to see if you actually like the skills you get. If not, then you don't have to play it.

At most I'd personally suggest one tank, one healer, one physical DPS, and one magical DPS because there's some lore quests you won't be able to see if you don't have those leveled. And if you do decide to go that route, level up scholar/summoner because you get two classes leveled up for the price of one.

But again, this is just a personal suggestion. You're free to level up anything you want. They got rid of the need to level up other classes for specific abilities (Like how warrior used to need gladiator at 24 just to learn provoke) so you can do just about everything combat wise on a single class