What is the academic and professional merit to correspondence courses when you already have a BA and 2 associates degrees?

You are maxed out on civilian ed, so look at it as a method to increase promotion point totals to gain you advancement. Also, correspondence courses are specific and technically oriented while your degrees are more broad based and not military specific.

Unless you have a BA in everything from the University of Infinite Knowledge

I struggled with this very question when I was an NCO. After completing some of the courses I had actually learned more about leadership than I ever did in college. If you are in a competitive mos then it will help add some promotion points as you have already maxed out civilian Ed.