When I first started writing, the most useful thing for me about being in a workshop or in a group of regular beta readers was receiving feedback on my own work. Not because I'm some sort of mad egomaniac who believes the world and all that is in it was made for her convenience, but because of where my skill level was when it came to evaluating my own work.
I mean, I knew if something I wrote really, really sucked, in the sort of "burn it before it spreads" way, but otherwise, no clue. All through Clarion, for example, I honestly did not know if my fellow writers would praise my story to the heavens and suggest I be given the Campbell immediately and by divine fiat, or if they would tell me to pack up my pens and go home. So listening to people tell me where and how my story was broken, or, if not broken, where it could be polished, tightened, and generally made better, was incredibly valuable for me. At that point, listening to what other people thought was the most helpful thing I could do for myself as a writer.
Somewhere along the line, in the past two and a half years, that's changed. I still have beta readers, and I still rely on them for nearly everything I write. I am still utterly grateful every time someone takes time out of their life to help me with my stories. But at this point, I can almost always identify the weak spots in my own work. I can't always fix them on my own, but I can ask for a focused critique.
Now, the thing that is most helpful for me is reading someone else's work. (NB: Dear people of the internets. I love you, lo, very. This is, however, not an open invitation to send me your work to read. I wish that I had that sort of free time, however, the sad reality is that I do not.) Beta reading for someone else, or looking back at a book or story that I've read before, and pulling it apart to see how the pieces fit, that's where I learn now. That's where I get my ideas for what I can do, and what I might try, and where I might aspire in the future.
I've been thinking about this a lot lately, as I have a student doing a creatively focused independent study this semester, and I'm helping a small group of students that I've taught this year put together their own writers' group. And I just keep being reminded that while writing is (unless you are collaborating) an individual and generally solitary activity, being a writer is not something that is done in a vacuum. We do need voices other than ours in our heads, even if the speeches change.