It has become something of a cliche, rather thoughtlessly repeated by well-meaning people of a certain generation, that to learn Thomism one ought to read Thomas himself and ignore the Thomist commentators and manualists who built on his work. I couldn't disagree more. No great philosopher, no matter how brilliant and systematic, ever uncovers all the implications of his position, foresees every possible objection, or imagines what rival systems might come into being centuries in the future. His work is never finished, and if it is worth finishing, others will come along and do the job. Since their work is, naturally, never finished either, a tradition of thought develops, committed to working out the implications of the founder's system, applying it to new circumstances and challenges, and so forth. Thus Plato had Plotinus, Aristotle had Aquinas, and Aquinas had Cajetan -- to name just three famous representatives of Platonism, Aristotelianism, and Thomism, respectively. And thus you cannot fully understand Plato unless you understand Platonism, you cannot fully understand Aristotle unless you understand Aristotelianism, and you cannot fully understand Thomas unless you understand Thomism. True, writers in the traditions in question often disagree with one another and sometimes simply get things wrong. But that is all the more reason to study them if one wants to understand the founders of these traditions; for the tensions and unanswered questions in a tradition reflect the richness of the system of thought originated by its founder.
-- Edward Feser, Scholastic Metaphysics: An Introduction.