everyone learns differently.
Some people can learn GC and Orgo directly with KBB and don't need much practice, just something in their brains clicks and helps them learn the material with as little practice as possible.... those people do NOT need the destroyer
However, if your like the majority of us, where KBB provides the foundation but not enough practice, then destroyer is the book you get. I don't find its biology all that helpful but the GC and orgo are the things that make destroyer book really shine.
Althought, if you do decide on the destroyer, do your self a favor and add that extra $50 to get both the DAT & MATH destroyers