I have beta'd for a couple of dozen people, and I have had others beta for me.
As someone looking for a beta, I think that there ought to be different terms: one for a reader who reads a completed ms that is as polished as you can make it, and another term for a reader who will read your stuff and help fix your grammar mistakes and things like that. I would never want anyone to correct my ms for spelling or grammar, but others might.
Maybe call one a beta and the other a crit partner, I don't know, but having clear terms might help everyone have clearer expectations. Clear expectations are important.
When I beta other people's work, I always offer to beta the first chapter or two, not the whole ms.
I give comments on the first chunk, and if people think my comments are useful and if they agree with the kinds of changes I suggest, then I will continue reading, AFTER THEY HAVE MADE THOSE CHANGES.
For example, if they have a lot of head hopping and POV confusion I'll make comments about it. If the author says, "I see the problem, I'll fix it," I say, "Great, send me the next chunk when you have done so."
If they say, "I don't see these things as a problem," then we disagree and there is little point in my reading the rest of their ms.
Sometimes they just have a different vision for their book, and my editing/writing style does not fit with theirs. I have read some good stuff from authors who rejected my suggestions for revisions because it didn't fit with their voice. That's fine.
But more often, I can find serious problems in the first chapter (bad dialogue, punctuation issues, said-isms, head-hopping, POV confusion) and the author doesn't want to correct any of it, they just want me to read the next three hundred pages so we can talk about the story.
The story may be neat, but it will never be published unless the author fixes these problems, so I feel it is a waste of time for me to read the rest.
P