Thoughts of the role of the Beta

Something moljn posted recently got me thinking. Her post is here, but I'll quote a part of it:

Do you have that thing where, sometimes, you start reading a fanfiction story and within five minutes it's clear that it's really bad fanfic? I mean, train wreck levels of bad, especially in the way you can't. stop. reading. [snipped] And then you check the story notes, because it might be their first fic, and it isn't, and then they thank their beta reader? And you can't believe the story had a beta reader? (honestly, it's the part with the beta reader that disturbs me the most)

I don't know what fic inspired the post, but my immediate response was: Well...yeah. But how bad do you think it was before the beta got hold of it?

( Cut for length )