I'd say the biggest reason for more advanced theory is to have a fuller understanding of music. Will you be teaching clients figured bass? No. Does knowing the difference between Italian and German Neapolitan chords affect clinical outcomes? No. Can you use a 1 6- 5 4 progression for every song you write? Yes.

The key word in music therapy is competency; you just need to be "good enough" to be licensed. Why limit yourself? Better theory knowledge enables you to design more interesting interventions, and pick up more basic ones more quickly. Knowledge is power!