The strong identity Plotinus maintains between the intellect and its objects, the ideas, can be explained in terms of his acceptance of certain sceptical arguments; in particular he holds that unless the subject and the object of thought are strictly identical, there is room for doubt and error. Moreover, I suppose that Plotinus believed that without this identity the traditional account of the forms as at once ontological and epistemological standards cannot hold. Thus, I am suggesting that we see Plotinus' position here as that of a Platonist who says to his fellow Platonists: if you wish to hold, as you seem to do, that among first principles are included both a universal intellect and the ideas, and that the ideas are to serve as the ultimate standards not only of created things but also was to some extent following a tradition, as regards this view he did not hold it just because any sane Platonist would hold it. Moreover, even if he took over the doctrine of the intemality of the ideas from his predecessors, Plotinus' version of it is quite distinct and goes beyond anything we can find in those predecessors.
Emilsson, Eyolfur, "Plotinus on the Objects of Thought" (1991). The Society for Ancient Greek Philosophy Newsletter. 182.