I mean, on any of these forums, what, five people might read what you post in the workshop?
OP finds it distracting or unhelpful to get broad picture comments; they apparently just want prose critique. Posting a shorter excerpt generally increases the number of people willing to read and comment, too. Seems like win/win. I'd say most writers tend to want much more than prose help, though.
You go, girl.
This is true. I'm not pretending like if you only do this, you're suddenly going to have an amazing piece. You
do still need people who will go and read it through from beginning to end because doing this is insightful, but limited. It will definitely never tell you whether or not your story is actually good or not. Or your characters make sense. Or the plot moves smoothly from point a to point b.
But it's a thing you're going to want to try before you go to the beta readers. It's meant to identify, as you said, issues with the prose. It's meant to identify bad habits. For example, if you're constantly spelling things wrong. Or you're stuck in using passive voice. Those are fixes you can then take from that one paragraph or whatever, and apply it to the rest of your story.
Then you can go to the beta readers, and they're going to have a much easier time identifying the things that they can help with. Like whether your plot is coherent and your character arcs feel resolved. Whether or not the ending hits the way you hoped it would. Which, in my experience, is very hard for me to focus on when a manuscript is full of serious writing issues.
I also found it opens up discussion from my writing a little more, which was something I've been needing very badly for so long. Instead of "Yeah this doesn't work for me" I actually got instruction on how to write it better. A lot of us don't have the resources to go to college or take a master class to do this. Some of us are using free software, for pete's sake! I think it's because they're not spending 2-3 hours going through your manuscript. So they can spend that little extra giving that little extra instruction on the fundamentals and why its preferred to do it one way instead of another.
I'm throwing it out to fellow writers as a suggestion to make a big part of your feedback process. It's less scary than a full blown critique. Less for you to get defensive about. It helps out your beta readers, because you're now cleaning up these little tiny silly mistakes that would otherwise be a distraction. And it makes it possible to fix your own work because you've been told how on that little snippet.