If it makes it any easier for you, classes are essentially going to be strictly "freelancers" a la FFV, which can mix and match any of their learned abilities based on the situation, whether it be soloing or playing certain light party content where it's more effective if everyone can hold their own for example, whereas jobs are simply specializations or "load outs" out of a squad-based FPS or something that will limit your scope of available actions, but make you much more effective in a given role for those moments when your party requires a much more organized layout and extra oomph for a full party raid or something.

Obviously if you're going to be stuck on one type of content, then you probably won't see the utility of whichever, but that doesn't mean it isn't there. Personally, I see class variety becoming VERY useful in PvP if done properly.