"Examine what is said, not who speaks" is an ideal that invites us to ignore any pattern of behavior that the speaker has demonstrated in the past. I've almost always found this ideal to be unworkable.

Through past behavior, a speaker demonstrates

• Competence
• Intention
• Perspective

all of which are useful to consider when holding up a new writing. When I'm pressed for time, I'll tend towards authors who've demonstrated competence and intention. (I can adjust for perspective.)

An extreme, but common, example: If someone has a history for skillfully crafted trolling, should a new post from them be held to a different standard than one from someone who never trolls? The ideal answer is "No. Examine what is said, not who speaks". The pragmatic answer is "Yes, of course. We don't want to waste time on another flame war, so let's apply extra scrutiny to this new post, and ignore it at the first whiff of troll."

I'll go with the pragmatic approach.