Nevertheless I think I invoke the principle regularly, particularly when working with obscure or fragmentary texts. There are often gaps to fill in or multiple potential readings of a given phrase and -- perhaps automatically now -- I try to go for the strongest, or best, or most plausible reading. But perhaps there is more than one role for the Principle to play.
First, the Principle has a role in simple philosophical debate and exchange. It is tactically justified. Here it seems to me that the Principle is not a two-directional thing. I mean: it is not the case that while I employ it when thinking about other philosophers' arguments I can assume that others will employ it when thinking about mine. For example, this seems to me like sound advice from Jim Pryor's useful guide for students writing philosophy papers:
No doubt this is in part a good and pragmatic dialectical stance: assume that you will be interpreted uncharitably and therefore make sure that you are clear and methodical. Of course, this sometimes makes for very dull philosophy papers which spend a lot of time rejecting pre-empted objections that no one but the most perverse critic would ever offer. But it is still a good practice. And, I suppose, if this is coupled with the original Principle of Charity it will make the stance even stronger: give your dialectical opponent the very best argument you can and assume that this opponent will be mean and uncharitable to you. Not only will you, if you refute the best option available to your rival, undermine in the process all other weaker versions (including probably your opponent's own genuine position), but you will also offer in advance a position already protected from as many mean and nasty criticisms as you can imagine. In fact, you can profitably take this one step further and pretend that your reader is lazy, stupid, and mean. He's lazy in that he doesn't want to figure out what your convoluted sentences are supposed to mean, and he doesn't want to figure out what your argument is, if it's not already obvious. He's stupid, so you have to explain everything you say to him in simple, bite-sized pieces. And he's mean, so he's not going to read your paper charitably. (For example, if something you say admits of more than one interpretation, he's going to assume you meant the less plausible thing.) If you understand the material you're writing about, and if you aim your paper at such a reader, you'll probably get an A.
Pretend that your reader has not read the material you're discussing, and has not given the topic much thought in advance. This will of course not be true. But if you write as if it were true, it will force you to explain any technical terms, to illustrate strange or obscure distinctions, and to be as explicit as possible when you summarize what some other philosopher said.
In fact, you can profitably take this one step further and pretend that your reader is lazy, stupid, and mean. He's lazy in that he doesn't want to figure out what your convoluted sentences are supposed to mean, and he doesn't want to figure out what your argument is, if it's not already obvious. He's stupid, so you have to explain everything you say to him in simple, bite-sized pieces. And he's mean, so he's not going to read your paper charitably. (For example, if something you say admits of more than one interpretation, he's going to assume you meant the less plausible thing.) If you understand the material you're writing about, and if you aim your paper at such a reader, you'll probably get an A.
This is interesting, though, because it is part and parcel of a dialectical situation which is certainly adversarial. The budding philosopher is being told to be clear and charitable in the face of an opponent who is mean and perverse.
Is this the same as when we invoke the Principle is studying ancient philosophers? I don't think so, since the second and distinct role the Principle plays is as an aid to the deployer of the Principle's own understanding. There, it seems at least in part not to be primarily part of a dialectical exchange between the interpreter and the text (though such an exchange will perhaps arise when we begin evaluating the reconstructed position) but rather a methodological principle tied to the difficulties of making sense of someone else's position, which may be expressed in a foreign language, may be based in geographically and chronologically remote cultures, and so on.
Yet another thought, this time even less well-considered: How is the Principle of Charity to be related (if at all) to a popular principle in historiography, famously outlined by Quentin Skinner?
No agent can eventually be said to have meant or done something which he could never be brought to accept as a correct description of what he had meant or done. How in practice this proposal is precisely to be applied is of course, no easy matter. But how is it related to the Principle of Charity? Does it place a constraint on what might charitably be offered as an interpretation of a given philosopher? Imagine if, in our eyes, the best version of some philosophical argument found in an ancient text is such that there are reasons to think that the philosopher concerned could not 'be brought to accept' this interpretation. Then how do we proceed? I suspect that answering that question will involve some grand thoughts about just what it is to 'do the history of philosophy' . But I can leave those from another day... Back to the marking!
 See also Richard Feldman's article on the Principle in the Routledge Encyclopedia of Philosophy.
 Quentin Skinner, 'Meaning and understanding in the history of ideas', History and theory 8,1969, 28. [JSTOR access here.]
 There are some interesting thoughts in R. Rorty, 'The historiography of philosophy: four genres' in Rorty, Schneewind, and Skinner (eds) Philosophy in History, Cambridge 1984, 49-75.