One thing we retain from studying a philosopher carefully is a general sense of what the philosopher said, as well as a general sense of what our reaction to it was. The details leave our memory after a certain period of time in most cases, but the most important points become hard coded into our consciousness in at least a general way. (For example, the details of Plato's Line analogy, which I just studied, will probably leave me in a few months or a year, but a sense of the distinctions he drew within the perceptual and conceptual levels will probably stay with me.)
This makes the principle that we should base our beliefs on evidence and good reasoning even more vivid and urgent than it was already. If you aren't careful enough in your evaluation of Kant and don't arrive at a firm conclusion about him that integrates all the evidence, you may find yourself, in five years when you have forgotten all about Kant, unable to spot the error in some Kantian argument or policy. It's not that you won't have the explicit knowledge about Kant, because you would have forgotten a lot of that at some point either way, but you won't have your psycho-epistemology set up and attuned in such a way that spotting the error comes to you naturally.
Moreover, your psycho-epistemology has a strong influence over what you will find interesting. For example, if you study logical positivism (without arriving at a strong evaluation of it), then you will find yourself more inclined toward ideas that are actively anti-integration. Studying something doesn't just give you more knowledge about that thing, it changes what makes sense to you, and that may lead you in a downward or upward spiral.
I think the main practical application of these observations is that we should try not to leave an evil idea in our memory uncontested. If you read something dangerous, you should make sure you know a good refutation for it before letting yourself forget about it, or you might end up acting on it or uttering it in an unguarded moment.