Why? The subject is rationality (def: based on reason), not validity and I think you must agree that the arguments and choices people make are rational to varying degrees.
I have responses yo your other points, but let's do it one at a time. Don't you agree that people make arguments and choices that aren't rational?
For an argument to be rational, it is necessary but not sufficient for the argument to be valid. Very few political arguments turn out to be valid.
Don't you agree that people make arguments and choices that aren't rational?
Arguments: fer sure. I only object to your suggestion that an argument can be a "partly rational." That's on par with "a bit pregnant" or "somewhat dead."
Choices? Is a choice ever "rational"? I think that in this context you are ditching the usual meaning of "rational," and calling a choice "irrational" if you disagree with it. Is my choice in music, food, clothing, a mate, or anything else "rational"? How do you identify the "rational" from the "irrational" choices? Choice is a product of human desire, not human reason.
I'd agree, though, if you'd asked, "Don't people often try to rationalize their choices?" Sure they do, but it's futile. The reason is always, "because I want to." No platonic argument can render a choice inescapable.