Why does everyone put black mage and monk together? For the white magic monk gets?
Really good synergy together. Dual jobs wasn't designed to begin with, so all you can do is look over all of them and see which compliments each other the best. Since monk gets access to some nice high level white magic, and a ton of HP lores, you have a super black mage which can decimate undead, along with most other types of enemies, plus provide massive cures.
If you want a more physical combat focused character, you wouldn't pair Monk with black mage though.
This is also why many people like Knight/Samurai (bushi) - the massive boosts to magic lores, and use of mage armor that bushi gets helps compliment not only Katana damage, but the high level white magic knight gets later on too.
At the end of the day, anything will work, but some combos are better suited than others for reasons like the above.(edit and below - wow)