Rather than argue about what would or wouldn't be useful and why, I'll just go back to your original question. Originally (version 1.0) player level and class levels were separate. At that time assigning attribute points when you leveled up, for instance, could be a difficult decision because those stats would be added to all classes. Similarly, new skills that you learned (pretty much all if not all of them) were available to be cross-classed with all classes. It could make for some strategic leveling choices. Personally I preferred to get things like cure and protect immediately.

In any case, interesting choices or not, what happened eventually was everyone would be more or less required to level every single class to be included in end game content. Regardless of whatever class you were actually playing you'd have access to every skill and so you could pick the 'best' way to play your class by mixing and matching them all. Eventually jobs rolled out and narrowed down what skills you could cross class while adding job specific skills and stats bonuses that made using a job sensible. This was to make the classes feel different from each other. Several skills were also updated at this time (and many other times).

When 2.0 rolled out they updated the skills again (and the entire combat system) and we still have vestiges of the previous system (like attribute points). While I wouldn't say the cross class system is obsolete, it's pretty stale currently and I'm not even sure if they intend to allow more cross class skills at level 60 or if any new skills will be added to the pool (seems somewhat unlikely).