No you do not HAVE to use the Specializations, its kinda like prestiging in CoD. You have the option to but you dont have to. Each Spec has 10 levels you have to go through, so if you dont want to you dont have to. But you unlock armor, armor skins, weapon skins, and a mod for each specialization you go through. So really besides taking the time to level through them, there is no downside