No, Sherlock doesn't need another brain. But he could benefit from an extra heart.

Sherlock: You're keeping a SCRAPBOOK. Only old ladies and pre-pubescent girls keep scrapbooks, John. John: It's not a scrapbook, Sherlock. I'm collecting papers relevant to the cases. It helps me remember the details. And it was locked away in my desk drawer. Sherlock: The lock on your desk drawer was insulting me with its pretense at security.