The T-600 was shooting inaccuratly with its minigun. It might have damaged the T-800. Or the t-600 was geting in the way of the t-800`s mission Also: It offered no aid in the mission from the point of view of the T-800. It could have caused damaged to the t-800 unit and generally got in its way. The t-600 was damaged and an "inferior" model, thus, not needed.