I don't think we know enough to know if Peabody "needed" his ink, or if the ink just made his magic harder-to-detect (and thus less risky, more pervasive), or what the exact mechanisms were.
Harry dismissively labeled Peabody a "bureaucromancer," and I think in some ways that isn't far off, and ink is a central feature of his magic. In the SummerKnight bit quoted above, Peabody appears to have used magical ink on himself. Presumably, a different kind of ink, but still ...