Instructor rating may be useful thing to have. CASA requires that Check and Training captains do a "theories of teaching"(or some other bullsh!t name for it) prior to approving C&T captains nowadays.
Also, if the goal is the RFDS... why not aim to be top dog there? C&T captains under the GA award earn an extra 10% of base salary if the aggregate of higher duties exceeds 90 days for the year.