On the whole it's probably true but how sad for principals who would therefore be the stressed doctors. There were five types of non-principal in general practice until about 1994. These were Locum, Retainee, Assistant, Associate and Deputy. In addition to these posts some health authorities are currently creating their own 'salaried doctors' schemes to deal with local recruitment difficulties. Locums work wherever they are needed doing either just short surgeries or longer stints.
Locums are normally self employed and experience the greatest variety of practices and patients.
Locums have the most flexibility in being able to choose where and when they work, but are also the most vulnerable when work is short. Retainees are doctors who work within the Retainer Doctor Scheme. It provides limited amounts of work for doctors with other commitments, usually women with children, enabling them to retain their skills and stay up to date. It is very restrictive at present and barely pays for the costs incurred. Revamping of the scheme is expected soon.