He shows how men assist each other to know the truth; for one man assists another to consider the truth in two ways--directly and indirectly.
One is assisted directly by those who have discovered the truth; because, as has been pointed out, when each of our predecessors has discovered something about the truth, which is gathered together into one whole, he also introduces his followers to a more extensive knowledge of truth.
One is assisted indirectly insofar as those who have preceded us and who were wrong about the truth have bequeathed to their successors the occasion for exercising their mental powers, so that by diligent discussion the truth might be seen more clearly.
Now it is only fitting that we should be grateful to those who have helped us attain so great a good as knowledge of the truth. Therefore he says that "It is only right that we should be grateful," not merely to those whom we think have found the truth, and with whose views we agree by following them, but also to those who, in the search for truth, have made only superficial statements, even though we do not follow their views, for these men too have given us something because they have shown us instances of actual attempts to discover the truth.
- St. Thomas Aquinas, Commentary on Aristotle's Metaphysics, John P. Rowan, tr. Dumb Ox (Notre Dame: 1961) p. 110 (Book 2, Lesson 1; Section 287).
I am certainly on board with the value of studying the history of philosophy. In fact my own education has been almost entirely from the "historical" perspective. I got my B.A. at St John's College, home of the original Great Books program, in which only primary sources are read, and the closest I ever got in class to a contemporary philosopher was Heidegger and Wittgensein, at the very end. And my graduate program at The Catholic University of America takes a similarly historical approach. And let's not forget my 450-page dissertation on an obscure corner of 13th century metaphysics! That said, history - even the history of philosophy - is not philosophy itself, and I couldn't really be considered a historian. As noted here a little while ago, while I largely study mediaeval philosophy, I'm really not a mediaevalist. Not that I think at all that these are bad things, and I would like to brush up my mediaevalist credentials - for instance, by studying paleography in more detail. But it's important to make distinctions about what you're doing, and scholarship is not the same thing as philosophy.
As with any virtue, there are two opposite vices to be avoided here. The history of philosophy is like a map. Setting out into the metaphysical unknown without it leaves you unable to avoid quicksand, tar pits, pointless detours, and dead ends, and you might never actually see anything of interest. Doing philosophy blind does not make you an intrepid explorer, it just makes you likely to go wrong. On the other hand if you examine the map simply as a set of geographical coordinates (or worse, an art object), and never do any traveling, then you can't be said to be doing philosophy at all. I know at least one eminent historian of philosophy who seems not to have any actual philosophical opinions of his own, despite being able to inform you with great accuracy and textual support of the opinions of others.
Now reading broadly in canonical (or obscure, if they're any good!) authors is certainly a good way to approach a philosophical question. Reading about a question tells me how other have approached and tried to answer it. But learning about how others have approached it is not the same thing as approaching it, any more than reading explorers' accounts is the same as exploring. One ought to do both, but remembering that philosophy is the love of wisdom, and scholarship, though it can bring knowledge, is not wisdom. The difficulty - because of course one would like to read every philosopher who has thought well, or even interestingly - is that if you wait until you've mastered the literature, you will never begin to actually do your own thinking. Mastering the literature can't be done. I can't read all of the good books of the past, and I can't even keep up with my contemporaries: there are too many of them and they write too much. But you have to start somewhere, knowing that you will never see everything you would like, and that you're not guaranteed at the beginning to end up where you would like. When I read one book I'm always choosing a particular dialogue partner and, for now, rejecting all the rest.
Knowing, then, that you can never master all of the thought of the past, at some point you just have to jump in and see if you can reason your way around a question. Do so without consulting a trustworthy road map at your peril, but remember that philosophy does not begin (or end) with footnotes.
Scholarship, however, does. If I do use a map, I'd better make damn sure I'm reading it correctly or it will do me no good. There's nothing more irritating than a philosopher pretending to be a scholar, using another thinker as a jumping-off point and then blithely distorting his position.