The big advantage of getting a CFI/CFII for the instructor is it will make them a better more well-rounded pilot.
The material that is covered in the CFI/CFII course was already taught to you in the instrument and commercial phases of your training.
The old adage goes... things that are taught may be remembered... if you can teach it to someone else then you have really learned the material. (or something similar to this).
You may be able to fly within standards using your techniques and procedures that work for you. Those may not work for a different pilot and learning and being able to adapt your techniques and procedures to fit another pilot is eseential from a great instructor.
When you get a job flying SIC in a turbojet, you will learn many new things and their techniques and procedures will be different than yours. If you have exposed yourself to as many different techniques and procedures before you get that job, the transition will be easier.
Even if you don't teach (I highly recommend it to make you a better pilot), learning the commercial and instrument theory from an instructional teaching standpoint will force you to learn the material better and make you a better pilot.