I'm trying to come up with a way to make an interesting faerie blood magus, with more than a mere +1 to a characteristic.
Can you gain something useful from sympathy traits?
Is there a way for a magus to gain something from having the profession/craft bonus, outside of joining verditius / rusticani?
There are many good ways to do interesting things with faerie blood. The bloodlines offering a non-characteristic bonus, such as an undine-blooded Aquam mage, or a sneaky goblin-blooded Tytalus, offer plenty of room for thought.
Going with gnome/dwarf use - sympathy traits are potentially extremely useful. You can apply the sympathy trait as a bonus to any ability in place of the normal +1 specialisation. As dwarves have weaponry as a sympathy trait, you can apply this to martial skills where you are using a weapon, crafting skills where you're making a weapon, finesse to use spells affecting a weapon. If you can find a sympathy trait that's reasonably close to spells you might use (such as a stone sympathy trait for a Terram magus) then using sympathy traits to boost Finesse can be amazing (to the point where some people on this forum have pointed out abuses of it).
Is there a way to gain something from a craft bonus if you're not a craft mage - yes, use the bonus to your profession:scribe, claiming that the ancient runecarving ways of fairy craftsmen means that your penmanship is exquisite, and you can copy lab texts and summae quicker.