Is it important to include opinions of historians in my essays?

Historical opinion - also known as historiography - is essential to essay writing. Including the opinions and arguments of historians shows your awareness of the field. It will impress your examiners if you are able to recite and understand where historians stand on a particular topic. An example could be the main reasons for WWI - although there will be some consensus, historians may disagree on the most important causes of the war. If you are able to convey to your examiners this disagreement in the field, it will make your work look well thought out. Understanding historical opinion also enables you to place yourself in the field: maybe you agree with 'Historian A' and disagree with the opinion of 'Historian B'. By including such information, it becomes clear what your argument is.