When developing the rules for this hack I didn’t really clarify what professions could be used for the defend action and specifically, in what context. Apologies. Some of the uses may have seemed obvious, e.g. using Acrobat to defend against Marksman but other situations may have left players and DMs puzzled.
By default Warrior is used to perform attack actions with melee weapons. It does not normally permit defend actions, this is the preserve of the Acrobat profession which governs manoeuvrability. Warrior rates proficiency with armour, an extra which in turn grants free invocations on defend actions (so there is a tangible benefit to the defend action under this profession).
If the PC is an experienced or well-trained fighter he can always select the Man-At-Ams stunt which allows her to use the Warrior profession to make defend actions against melee attacks. See the Professions section in the Dungeon Master’s Guide for the full description.
Just as with the skills described in the core rules, some of the professions allow you to defend against attempts to harm your mind or damage your resolve. Mystic would typically be employed to defend against attempts to charm (Aristocrat), deceive or provoke (Rogue). Or, if the action is calling for active opposition you might find your character employing Aristocrat to defend against Aristocrat in a negotiation over a business matter. Equally it might be a case of using Rogue versus Rogue where two characters are bluffing each other in a card game.