“I’ve never understood why Susan Decker is on the board……….”
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I also have questioned her commitment to Berkshire based only on her BRK share ownership.
Many on this board have an order of magnitude & greater skin in the game!
The following search result may shed more light on her value but I still would like to see her up her ante in Berkshire to show greater commitment.

A quick search on Gemini regarding Susan Decker’s value to BRK board of Directors ,

“Role as Lead Independent Director
Shareholders generally view her role as Lead Independent Director as a critical stabilizing force for the post-Buffett era.
• Succession Planning: She is tasked with providing recommendations on board size and composition if Buffett can no longer serve. 
• Expertise: She is officially designated as an "audit committee financial expert" in SEC filings, a credential that provides comfort to institutional investors regarding the company's financial oversight.”

ciao