why people choose to cross job than be a pure job?

The reason people choose a cross job class rather than be a pure job class is because a cross job class character have more advantages than a pure class character , example : ranger (accretian) crossing to mercenary , the advantages is he got evasion , & counter and his total evasion is 196 it is if his elemental is perfect (30.25 A.A ) , and his has more blood (HP) than a pure striker or a phantom shadow ,and  the other advantages is he has six skills , from ranger 3 , from warrior skill 3 But the disadvantages that he have is he can't have a skill that a pure job gets , such as elemental resist that every mercenary has , or a siege mastery that every launcher has.

The example of crossjobing character is on the image
It is a Ranger class crossing to a Warrior class .