Even if your client is not actively trying to build his practice, grabbing his social media sites is still recommended. What happens if another person grabs those names and begins using them? It can cause a headache for your client to explain to his/her patients the social media site is owned by someone else.
Worse, what if the owner of the social media account offers bad advice or otherwise acquires a negative reputation. A facebook or twitter page with the same name as the doctor's office could build the page to rank higher then the doctor's website is SERPs.
Another consideration is connecting with patients. You can reach out to patients via phone, mail and e-mail but people move and many may not remember to update contact information with their doctors. Social media allows patients to connect with the doctor rather then having the doctor's office to connect with the patient. A doctor's address/phone number change, vacation and other information can be communicated once via social media and reach many patients who otherwise might be missed.