Low level dungeons could be worth the effort if they rewarded something like an item that would give you a XP gain buff for like the next 10 levels. The buff could stack with unique kills, so if you kill 3 bosses in Scarlet Monastery, you get a level 3 buff. Re-killing the same bosses doesn't affect the boost.
Or even simpler: Fill in the gaps in the heirlooms, like belt, gloves, boots etc. Last boss of each dungeon drops a BOP filler with exp boost. Doesn't scale and will be replaced by a stronger item, so the incentive to do the dungeons 10 levels up is there as well.