Many historians today (read: my thesis advisor) have a bias against biography, both in the ancient, moralistic sense and in the modern, personal narrative sense. In the purest sense this is absolutely the right move, sense a simple narrative of events is no more a great historical work than is a newspaper and the moralistic view of biography is meant to teach ethics through the portrayal of certain events. On the other hand, when these parables are all that exist, that is what you work with.
Simply in reading biographies or pairs of biographies of Plutarch, one doesn’t get the sense of how much of an issue it is. The more of Plutarch that is read, however, the weaker he becomes as a source. In each instance the parables and quotes and discussions fit, but in reading the sayings of Spartans, almost all of the famous quotes are attributed to two, three or even four different speakers, and moreover, Alexander’s famous line of “So would I, were I Parmenion,” is attributed to a Spartan at another point. More than anything this repetition highlights in Plutarch a weakness in his sources writing so far down the line.
Of course from this period few of the sources were truly informed on their topics because, while we often see ancient as ancient as ancient, authors often wrote on topics hundreds of years in the past. To stick with Plutarch, his life of Alexander was written about someone who died 400 years before his birth. This, for me would be like writing an intimate book about Sir Francis Drake, only to have it be considered canon on the grounds that we lived within the same half-millennium.
When I addressed Plutarch as a scholar, I took great pains to acknowledge the weaknesses of the source, as I did with the other sources and at several points in the introduction and the conclusion, but I also felt obligated not to dismiss it out of hand. Sources are too scarce and as the root for history derives from that of arbiter, so must the modern eye take a skeptical turn and each address the validity of each source for themselves.