The three major transformations of a Guardian Beast are Instrumentalization, Attachment, and Fusion.
Instrumentalization is the most common among all Guardian Beasts, referring to a specific trait that allows a Guardian Beast to be transformed into a weapon.
According to ancient records, this particular trait was especially prevalent in the fourth, fifth, and sixth epochs.
There were no mentions of spiritual instruments in the scriptures of those times, and it is for this reason that Instrumental Beast Masters were especially popular.
The spiritual instruments produced by Spirit-smithing Masters possessed functions similar to those of Instrumentalized Guardian Beasts.
More importantly, a Beast Master could freely switch their spiritual instruments to suit their own combat system.
Instrumentalized Guardian Beasts could not offer this degree of flexibility.
Therefore, now, there's nothing particularly exciting about contracting a Guardian Beast that can Instrumentalize.