The Dragoon job is different from the Lancer class. You can cross-class any cross-class abilities as a Lancer. However, as a Dragoon, you can only cross-class Marauder and Pugilist skills. This is to emphasize the point that Jobs are specialized roles for tank, dps, and healer roles, while classes are more generalized. Classes are also being phased out in general and replaced with Jobs (because there's no reason to force players to grind through 30 levels for something they won't even use past level 35).

Cross-class abilities are at best for utility. Do not expect skills to give actual class mechanic benefits as it will be nerfed out. There is a reason blizzard does not give healers umbral ice (infintie mp).