The Umbrella Corporation wrote:
Kirk needed his main team on duty when things got critical.
In term of running the ship, who were the "Main Team?" Lt Palmer in terms of ability was probably interchangeable with Lt Uhura. The officers at the engineering station on the bridge switched around regularly. Sulu and Chekov weren't alway the two at helm/nav.
Kirk and Spock might camp out on the bridge during a on-going crisis, but there no reason that the rest of the bridge crew wouldn't maintain a fairly normal shift rotation.