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Flynn isn't only valuable as a witness for his prior access to Trump, but also because he's a former intel officer with the knowledge necessary to have made expert observations about Moscow's interference, its gradual compromise of Team Trump, and any collaboration between them.
It's not that he would testify as an expert, but that he may have been able to observe important details that others may not have noticed due to his professional skillset. He can testify to these details as a witness.
