If we think about it, we can discover a great lesson from the Torah: the concept of having someone to go to for guidance. As Rabbi Twerski explains, showing your flaws to a Kohen should help you want to change them, because of the embarrassment. Another advantage is that if we have challenges that are hard for us to overcome, it would help if we talked to someone who might be able to guide us. In this case the expert was a Kohen, but if a suit of ours got dirty we would take it to professionals to clean, and we may even point out the stains. By the same token, we should treat our souls the same when cleansing ourselves of bad habits (both Halachic and personal), and a Rabbi happens to be the expert in the Biblical field. And the truth is that a partial healing is enough to purify the stain because it shows that there was effort to change. The lesson of the Kohen and the leprosy is just as our sages advise us in Pirkei Avot: find yourself a Rav (Rabbinical authority that you're comfortable with). In the end, we shouldn't be ashamed of our weaknesses unless we're doing nothing about them!