Only Careless Writing
...you casually toss out “I don't pretend to follow everything Nietzsche is saying in this book but he seems . . . to be impatient with the dishonesty inherent in most philosophy (emphasis added).” Whoa, what? Is that something you think he believes, or are you tacitly agreeing that that is the case?
You were right to pick up on that phrase. It absolutely should have read "...with what he sees as the dishonesty inherent in most philosophy.” I'm tempted to go back and make that change but, alas, I've been quoted now. No, I don't think that I agree with him, although he makes compelling points about certain specific cases.
Your third paragraph is well put. And it's a good question: what does it mean if we agree that systems of thought are only justifying current beliefs? I think you're correct that the right way to look at them is mostly as good faith efforts to understand why we believe what we believe regardless of whether we agree that they're "right" or "proven" in the way a law of nature is proven. On the other hand I see his point in not wanting to ignore the moral influences that led someone to their belief system especially when, as with Spinoza, they present that system as having been arrived at through first principles.
Either way, I don't want to walk myself into a rigid defense of Nietzsche. I'm remembering midway through this book what made me quite uncomfortable at times with his writing, with his talk of "the mob" and "will to power" and misogyny. So, putting aside what he thinks of other philosophers (tldr mostly not a fan), it was his statement about listening when the cynic speaks that really jumped out at me originally. For reference
in brief, whenever anyone speaks "badly" of man–but does not speak ill of him–the lover of knowledge should listen carefully and with diligence, and he should in general lend an ear whenever anyone speaks without indignation.
If I read your last paragraph about pessimism correctly (and your earlier comments about ancient tweets) then this jives doesn't it? I'm going to again attempt a clumsy summary of what he's saying that may get me into trouble: good or bad, moral frameworks are human creations that often simply reflect the author's (and his/her society's) beliefs at a point in time. It's useful then to listen to the cynic, who is making unflattering and unpopular observations based on what is arguably constant about mankind. These would be the people not inclined to "believ[e] things can never change that much that quickly".
I have to admit that I've probably been blending the ideas of cynicism and pessimism in my head (in general, it's always good not to rule out careless writing as a possible explanation for confusion in my posts). I'm not sure if this one has helped to clear anything up but, for what it's worth, I don't think I would ever be inclined to attack pessimism. To be clear, I am often both cynical and pessimistic. My objection – my impatience – is with our whole environment being flooded with criticism and condemnation and paralyzing fatalism. Put another way, the guy on Twitter lamenting how broken democracy is may not be wrong, but I don't want to read about it, I don't know that it's helpful information, and I think it may be making things worse. But Nietzsche (and maybe you) are possibly saying that – no, it really is worth saying and worth listening to.