Most EC's are a complete waste of time.
A pattern of related, committed EC's can show dedication.
Example: I want to go into emed, so on top of 4th year elective rotations in emed, I would also choose dive rescue, ACLS, pediatric life support, ITLS, and tropical medicine selective courses during my basic science years.
The Integrative Medicine selective I'm doing that has lectures and workshops in Yoga, Karate, Chiropractry, Soul Retrieval, and Acupuncture? It's fun, but not really useful.
Joining the MPH, PHR, AMSA, and other random service-related societies that do volunteer projects? That could show a commitment to public health.
Joining the Large Animal Society, the Neuroscience Club, and Women in Medicine (as a guy)? Not so useful. But hey I got t-shirts.