Another entry in the "Mystics turn all slots into Arcane slots," this is a cheap, fragile way to get an extra spell into your rig.
What can't it hold? Empower Self, Empower Self, Empower Self (either singly or as a set); Enchanted Blade, Enchanted Blade (3), or Enchanted Blade (3); or Mind's Eye Since it's a level 0 Mystic, the Familiar can get around and refuse to hold: Arcane Enlightenment, Borrowed Time, Double, Double, Dream-Enhancing Serum, Farsight, Flesh Ward, Haste, Rite of Sanctification, or Spiritual Resolve. This cat is sort of finicky.
The Familiar is cheap, it plays into a low-resource deck that wants to pay for spells with Robes of Endless Night and Shining Trapezohedron. It is also Fragile, easy prey for Ally-sniping Treacheries or Location effects. It occupies the hotly-contested Ally slot, but a low-resource deck can get by without David Renfield, so maybe that slot became a little less contested. It might help keep a Sixth Sense or other spell available while your own Arcane slots are full of Mind's Eye.