Having played Hannah and librarians a lot (in relation to the original question) I find the counterspell most useful against: Neverborn, Ressers, and Outcast (Daw, Levi, Tara, and Hamelin crews all have potentially multiple cast spells, not to mention any that have their own Hannah or Librarians).
Other factions have at least one cast master per (Sonnia, Marcus, Raspy. Wong, etc).
Overall I find it a very useful ability to have, but wouldn't add it to a crew specifically unless one of the 3 factions I mentioned above were announced.