David O. Ross, Jr., Virgil's Elements: Physics and Poetry in the Georgics
(Princeton: Princeton University Press, 1987), pp. 29-30:
One last point of introduction: Servius' commentary will be quoted frequently in what follows. I have come to find Servius of far greater value than anything written on Virgil since. Servius' commentary is a compilation, a variorum, part of which must go back four centuries to comments of various sorts made soon after the first publication of Virgil's poems. (For this reason I have not found it worthwhile here to attempt to distinguish consistently between Servius and Servius Auctus—the Donatus commentary.) These notes are often valuable in two ways. First, they illuminate because so frequently they provide us with a set of ideas entirely different from our own, an entirely neglected view of a word, or phrase, or passage that proves reasonably to have been Virgil's intention, long forgotten or overlooked simply because his own terms had been forgotten; understanding the Georgics is so often a matter of seeing the exact terms used in any passage. Second, even when a comment seems to us trivial, unnecessary, or silly, it often indicates that in antiquity some problem existed, that a line that seems clear enough to us seemed then to call for explanation, either because we simply don't see what bothered an ancient reader, or (as I suspect may frequently be the case) a comment, good and informed in itself, elicited another (but trivial) comment later, after which the informed comment dropped from the tradition, leaving only the dross to mark the place. I cite Servius, then, whenever I feel that what is there recorded provides a possible indication of the truth, known once and since forgotten.