I have put out "open calls" twice, I think.
The first was immediately after I joined, before I knew anyone and anyone knew me, when I didn't know anything about self-editing, when I had no idea how much I didn't know. I got responses, but quickly lost contact, and not much came of that particular "open call." Rightfully so, I think. My MS sucked butt.
The second time was maybe last year sometime (I can't remember), after I'd been around, people had seen me, I'd interacted, worked my butt sore to improve, been present wherever I felt I could be, critted, beta'd, had pieces in SYW, etc. I got several responses from people who knew me, who I knew also, who I'd interacted with, who gave me thorough and fantastic critiques. People who I might have asked to beta directly if I wasn't such a wuss.
A lot of it is about trust and familiarity. I have offered to beta for people on blind (or near-blind) whims, because I do have these lovely flashes of whimsy, but it's rare. A bit like blind dating, I guess.