You should expect people to try to prove what they believe! I also find the "I found X safe to ignore at Y point because of Z" to be a rather poisonous take on processing other viewpoints. I'm sure you mean well with it but you come off as looking for excuses to be dismissive.
I'd agree up to a point, but if I read something where early on there is an obviously biased and/or incorrect series of statements being made, it's not much of a stretch to assume the rest of the content will share that trait. Still, it doesn't necessarily mean they won't make some decent points regardless, but it makes it all too easy to dismiss an argument when it's formed that way. I'm not sure I'd call it "looking for an excuse to be dismissive", as much as "not giving the author a chance to redeem their argument in spite of obvious weaknesses".