That's not how light arts works. It sets the skill to a certain level based on your main job's level. war/sch will get over 300 healing skill as /sch, even if sch's skill normally caps at 140ish. whm/sch with 350ish healing skill will have their healing skill pushed to around 400 with light arts.

Additionally, all magic in this game has retardedly low skill up rates if the target is a player character. Magic only skills up at a decent rate if the target it an NPC of some sort. Luckily, you can cast cure on undeads and deal damage, so that would sort out healing magic skillups. Enhancing isn't as easily dealt with, as you will have to find friendly NPC targets to spam on if you want that skilled up. Besieged NPCs work, your adventuring fellows do as well.