[My worries about history requirements reminded me of this old draft from a couple of years ago that I never got around to publishing. Better late than never, I suppose.]
It often happens that a reader "takes away" something quite different from a piece of writing than the original author intended. Does this matter? I think that the answer is "sometimes", but I'd like to get clearer on precisely which times those are.
Let's say a blogger goes away and writes up a response to the idea X they "took home" from another writer W, whom it turns out really meant to say Y instead. I'm wondering: in what circumstances is the latter fact relevant? This seems to turn on the further question: is the blogger engaging with idea X for its own sake, or are they instead trying to respond to whatever person W might be saying? Which should they be doing? Here are a few cases where engaging with the person (and their actual claims) seems important:
1) If you insult or dismiss W on the basis of what they're (allegedly) saying.
2) If W is specifically trying to engage you, say by offering an objection to an earlier argument of yours.
3) If you're in a forum where W gets to choose the topic, e.g. commenting on their blog post.
Are there any others?
Those cases aside, I'm partial to the "disembodied ideas" approach, myself. Of course, it's worth listening to W all the same, because this new idea Y might be more interesting-in-itself and worthy of your attention than X was. But if not, that's fine too. As a general rule, the intellectual interest of an idea shouldn't turn on which particular people believe it. (Though I guess winning the support of a reliably discerning person might constitute 'abstract'- or meta-evidence that an idea is worth a closer look.)
Public interest may be another issue: of all the possible bad arguments out there in logical space, we're usually only interested in refuting the ones that are (or threaten to be) actually taken seriously by a significant section of society. Even so, the Writer's personal beliefs don't seem directly relevant here. But perhaps egregious misreadings are. That is, if nobody else is likely to interpret W the way you did (as arguing for X, say), and no-one else has defended X either, then arguing against it doesn't serve much of a public purpose. But hey, not everything has to: if you found it interesting to clarify the issue in your own mind, or whatever, then that's fine. Let a thousand flowers bloom and all that.
To address the flip-side: how should we, as writers, react when others offer false "responses" to our posts, e.g. criticising claims that we're not really committed to? Again, it may depend on the particular situation, but it seems like the ideal would simply be to clarify your position without forcing the other person on to the defensive. (E.g. "Note that my post merely meant to establish Y, which is consistent with your denial of X. So I don't think we really disagree here.") Focusing on the assessment of disembodied ideas seems more likely to lead to a pleasant exchange than some of the more conflict-ridden person-involving alternatives.
(Which is not to say that I always live up to these ideals, of course.)
P.S. A related question for historians of philosophy: is it intrinsically important to discover what old philosophers "really" meant, or should we use them more instrumentally, to garner whatever interesting ideas they might suggest to us? (Or do you think that the historical task is essential to the instrumental one?)