OTOH, if you manage to get dungeon closer to your actual level, you get the same bonus EXP, but also with a much better exp per minute rate.
Bonus exp from lower level dungeons scales to offset lower EXP in the dungeon. From what I've seen, the lower the dungeon level, the higher the exp bonus. The biggest bonus I've seen was from Sastasha (175K on 42 WAR).

If anything, the experience from Primals is too high in low level DF, and needs to be brought into line with the time vs. reward offered by the dungeons.

Apart from that Duty Roulette rocks, it's made leveling lower level jobs so much more enjoyable that it's ridiculous.