I've actually had sessions with three practitioners, two are instructors. I haven't asked the third, but two of them said they only do the body sweep once, at the beginning of the session.

Then they just do their breathing the rest of the session.

Did I miss something in the book? Isn't it better if you're doing sweeps with your breathing throughout the session? Or after a while advanced practitioners don't need to do the body sweeps?

Yes, you probably misunderstood. Sweeps are to be done throughout a session and they clearly amp up the energy. Keeping the sweeps going is an essential part of getting the full benefit of the basic QT technique.