Usually you'd level past 15 for cross class abilities. You can see what abilities other jobs can use if you go to the actions menu. It lists all class actions through 50 and in the tool tip it says what classes/ jobs can use it. Most of the useful ones are lowish level, but a few (like stoneskin and blood for blood) require a bit more leveling.

Leveling a second, third, etc. class isn't too bad really. You won't be getting experience from maps, main scenario, or likely much from side quests, but the dungeon roulette offers a great exp bonus once daily and dungeons give good experience by themselves. Hunting logs give a modest but easily acquired amount of experience, leves are easy and more or less decent for experience gains, and FATES can be good depending on the zone, level range, others' participation, etc. For a DPS it might be a bit of a chore, but for tanks and healers you can pretty much repeat dungeons as often as you like with minimal down time.