So much is going on in Plutarch's De genio Socratis that it's probably an error to think of the dialogue as having only one end or theme. However, I would suggest that a good first approximation in interpretation would take seriously the idea that a major theme of the dialogue is that of interpretation of evidence for the purposes of making decisions. The dialogue is constantly talking about this. Not only do we have the recurring discussions of Socrates' divine sign (in the three interpretations of Galaxidorus, Simmias, and Theanor), but we also have recurring instances of sacrificial omens (especially the opposition between Hipposthenides's interpretation of the sacrifices of Ceres and Theocritus's interpretation of his own sacrifices) and the various signs associted with the opening of tombs, as well as some indication of divine oracles. Nor is it only divine signs that are in play here. The conspirators have to coordinate with the exiles by messages and they have to assess the state of their conspiracy by reading the political signs, including those that suggest that the people in power already have wind of the conspiracy. Archias blunders by failing to read a message from Athens. At several points, people miss some key event and have to guess at or reason to it on the basis of what evidence they do have. It is specifically suggested at several points that the reading of divine signs is just a more finely tuned example of what we all regularly do in reasoning on the basis of our experiences. We swim in an ocean of evidences; signs are everywhere. All our decisions depend on skill and good judgment in the interpretation of signs.
Closely linked to this are the clear indications in the dialogue that personal character plays a significant role in interpretation of signs. It is especially good people who are given the aid of divine signs. Archias's wantonness leads to his failure to read the essential message on which his government depends. Hipposthenides without any malice misinterprets signs due to the weakness of his nature. And perhaps most significant of all is the character of Epaminondas, which adds an additional layer of complexity to the meaning of the dialogue.
Epaminondas was one of the greatest generals of the ancient world. After the liberation of Thebes, he would go on to do what even a generation before would have been thought virtually impossible: he broke the apparently unbreakable military might of Sparta. The Battle of Leuctra is one of the major case studies of military history, and it would be by building on his tactical innovations that the Macedonians under Philip and Alexander would change the face of the world. He is a significant character, of course. Unfortunately, we cannot compare his presentation in this dialogue to that in Plutarch's Life of Epaminondas, since the latter has been lost, but we do have Plutarch's Life of Pelopidas -- Pelopidas was Epaminondas' best friend, one of the exiles, and the one who struck the killing blow against Leontidas. In the Life of Pelopidas, Plutarch puts great emphasis on Epaminondas as a philosopher, which matches his portrayal in the dialogue on Socrates' daimonion. Epaminondas was a student of Lysis the Pythagorean; he devoted his life to philosophy and the cultivation of the mind. Plutarch portrays him in the Life of Pelopidas as having a wide array of virtues -- fortitude, magnanimity, gentleness, temperance, and justice, all of which are also on display here. In addition, in this dialogue Theanor insists that he has inherited the daimonion of Lysis, or at least some similar one.
This brief but vehement insistence by Theanor is all the more significant when one considers that Epaminondas is not portrayed as fully in favor of the revolution. He does not disagree with his goals, and has been working toward that end. In the Life of Pelopidas we have an interesting example of this:
Epaminondas, too, had long since filled the minds of the Theban youth with high thoughts; for he kept urging them in the gymnastic schools to try the Lacedaemonians in wrestling, and when he saw them elated with victory and mastery, he would chide them, telling them they ought rather to be ashamed, since their cowardice made them the slaves of the men whom they so far surpassed in bodily powers.
However, he refuses to participate in the conspiracy itself because of the danger of injustice toward his fellow Theban citizens -- a danger that becomes real in the assault, since a number of Thebans end up dead even despite the efforts of the leaders to persuade them at least just to stand by. That Epaminondas opposes the full conspiratorial plan is partly due to the fact that Plutarch is working with a historical story, in which Epaminondas only helps to consolidate the liberation of Thebes, but his philosophy, his virtue, and the claim that he is guided by a daimonion all contribute to showing that Plutarch does not intend the conspirators to be regarded uncritically. Unlike Epaminondas, who always acts with equanimity, most of the conspirators are heavily influenced by their passions. That the passions distort our interpretation of signs and lead us even to overlook essential ones is made explicit by Simmias and shown vividly by the fall of Archias. It is the fact that Socrates has purified himself of anything that might enslave himself to the passions that makes it possible for him to recognize what his daimonion wishes to indicate. Epaminondas, too, has trained himself to rise above his passions; he even clarifies exactly what this training was in his argument with Theanor about whether he should accept Theanor's gift: Epaminondas argues that we should train ourselves in virtue by foregoing the satisfaction of even acceptable pleasures, when they are not necessary. Epaminondas interprets signs well because he has the character for it. The conspirators have the right ends in view; but their means guarantee that they are continually in alarm at the possibility of discovery, and continually in danger of overreacting.
This emphasis on character, incidentally, is almost certainly one of the links between this dialogue and Plato's Phaedo, since the latter explicitly makes the case that the philosophical life is a kind of dying to the passions in order to achieve greater intellectual contemplation. I think it is clear that it is this purification from passion, not Socrates' death itself, that Plutarch primarily has in view in building the obvious links between this dialogue and that one.
What we learn from the dialogue, then, is that we live in a world filled with signs, both divine and otherwise. Proper interpretation of signs, however, requires the cultivation of a philosophical character, like that of Socrates, like that of Epaminondas. This is relevant to the making of decisions. Galaxidorus notes the fact of human psychology that we are often in some doubt about what we are to do, and that in this case a minor sign that would otherwise not sway us may actually tip the balance of our decision. (Simmias will deny Galaxidorus' claim that Socrates' divine sign was just a rule he had discovered by experience and inquiry, but everyone in the dialogue recognizes that small things can make massive contributions to decisions.) It appears to follow from this, however, that having a sense of what signs to follow and how to interpret them is part of the moral life: whether you do right or wrong will depend in part on how you take the little signs and evidences with which you are inundated. The passionate natures of the conspirators are constantly seeing causes of alarm; Epaminondas sees instead when it is appropriate to act, or to avoid acting, in order to act justly and with restraint.
Quotations from De genio Socratis are from William W. Goodwin's translation of Plutarch's Moralia, at the Perseus Project.