Charity sounds like a good thing. And it sounds like a good thing when reading the history of philosophy. If a philosopher seems to be incoherent, or self-contradictory, or if the argument seems problematically gappy or to admit of obvious counterexamples, or if the philosopher seems to be saying something patently false, we should still try to put the best possible light on the work. We shouldn't take the text at face value but instead plunge deeper, looking for the real insight beneath the surface.
Another reason favoring uncharitableness and superficiality involves a certain sort of externalist or public view of what philosophy is. Philosophy is not, I submit, primarily a matter of private performances of profound insight. It is a public act of stringing together words and arguments. What is on the page is the philosophy, regardless of what the philosopher might have thought in his secret heart. So if what is on the page is nonsense or self-contradictory or plainly false, the philosophy itself is nonsense or self-contradictory or plainly false. In fact, without the structure enforced by committing one's views to words, philosophical thought tends to be amorphous and weak. Thus, especially in light of the first consideration above, it's a dubious conjecture that beneath the philosopher's surface incoherences are hidden diamond arguments, only poorly expressed. Philosophical thinking is for the most part, and at its best, an outward act of explicit writing or speaking before others.
A third reason to avoid excessive charity is this: What you think is plausible or obviously true might differ enormously from what other people, especially in different cultures and times, think is plausible or obviously true. The reader of philosophy risks hiding this from herself if she insists on de-radicalizing and commonsensifying the texts she encounters, transforming Hegel or Plotinus or Ibn Rushd or Laozi into some approximation of a 21st-century New Yorker. Much of the value of reading history is lost if we insist on turning a blind eye to what seems to us to be obviously crazy -- partly because what we currently think of as "crazy" might in the end be true and partly because, even if it's not true, it's a salutary and humbling exercise to appreciate the wide range of bizarre things that philosophers have believed across history.
I hereby resolve to remain, until further notice, a superficial and uncharitable reader of the history of philosophy!