It’s not a bad idea, but it has a lot of repercussions that are not desirable.

1) You could level jobs you should be playing and knowledgeable on how to use (at least you must level the last 10 levels worth even if story skip).
2) Players would stockpile the boosts and insta cap jobs as new expacs launch.
3) It is against their own best interest (to encourage playing a variety of jobs in the actual content, and to sell their level boosts).