Well, practically speaking, it is a higher-skilled job... the game equivalent of university as opposed to trade school. Here's the incentive for NPCs to not become sages.
A level 1 Expert can take 4 ranks in one Craft skill, and make 3.5 - 4 Gp per week (depending on their Int score)
Alternately, they could take 4 ranks in two Knowledge skills, and make 2 Gp per week.
In summary, a marginally competent craftsman makes about twice as much as a student sage.
The attraction of the job is this...
A level 10 Expert craftsman can make about 6.5 Gp a week, with 13 ranks in a craft skill.
A level 10 Expert sage (or any other equivalent sagey job) can make 3.25 Gp a week... per Knowledge skill they've maxed out. So a level 10 sage with two knowledge skills makes 6.5 Gp per week... the SAME as a level 20 expert craftsman... but if he's got 3 skills, he makes 9.75 Gp, and so on and so forth...
This is balanced by the fact that craftsmen get more CP... the aforementioned craftsman gets 13 CP per week, while the sage gets 2. (not to mention that Magecrafters get even more of both, assuming they're ultilizing their Wizard feats towards crafting).
Therefore, the sage job is really quite balanced as written... what it does is reward focus. The more skill ranks put towards knowledge, the more valuable it is.