I imagine you want to avoid being a sort of "jack of all trades, master of none" at least in group play. While a healing ability might be great help for a damage dealer during solo questing, in group play players are expected to specialize rather than offheal as a damage dealer. Why? Because you offhealing means you are doing less damage, which is the reason you are in the party in the first place. So it is just more efficient to leave the role to the class that is most proficient at it. So I would suggest you pick a Job you like, and then focus on getting the cross-class abilities that help said job.

That being said, it doesn't seem you can really "ruin" your character. Level what you enjoy, and at worst you'll have to level some other class for the abilities you need for more competitive gameplay as well.