The cost approach is important (even when you think it is non-sense or not applicable) because by breaking down the property into land and building components you can create many issues that would not be relevant in the other approaches where land is not separate from the buildings.
Components of the CA also are the building blocks from which the HBU analysis begins (land value, as if vacant).
This might seem hokey, but I took a cost approach class online recently, mostly for the hours. I have over 30 years experience, SRA, I teach residential appraising, and am a St. Cert. General, so my "arrogance" allowed me to think this class would be a whiz and trivial. I learned SO MUCH from this class, and it is amazing how strongly the CA ties in with the overall valuation process, EXCLUSIVE of it's indication of market value. It is one of the best classes I have ever taken, because I had forgotten much of what I may have learned, and never learned near enough to begin with.
No, I am not a affiliated, but it is by Appraisal University. It is taught by an MAI (I Believe), and if you take it and tell me you did not learn anything (let alone a bunch), or if you do not walk away having gained a very strong respect for the Cost Approach (NOT necessarily as an indicator of market value but as an integral part of the appraisal process) I will, well, I will be surprised.
BTW, you get 24 CE hours, certified in most states, and several GOOD other classes (CAN, you would like the income property class, it is EXCELLENT), all for something like $129.
I bet MOST appraisers (designated as well as non) have never taken a dedicated Cost Approach Class. I don't think the AI even teaches one. Thus the general lack of knowledge of the process...