I think the biggest advantage to online writer's groups IS time management. Trying to get five or six people together in one spot is not an easy task these days, especially if the people in question have kids. Schedules are just too tight. Online groups mean people can participate whenever they have a spare minute in their day.
That said, I'm not big on them. I read for fun and I write for fun. Reading for a purpose (to critique) generally means I don't really enjoy it; reading should never resemble a job for me. And almost by definition, the stories aren't that good. The authors are asking for help to polish them, acknowledging that the story needs help. I have so much to read that IS good, I find it painful to slog through bad stuff. If it were just a bad story in a magazine or a book, I'd be free to put it down and walk away. In a critique group, the whole point is to read and give helpful tips. I can't do it. I've tried.
Also, you can run into the people who submit because they want the ego boost. They want to be told their work is sunshine and roses, and the least little bit of criticism results in temper tantrums that would make a two-year-old blush. Or worse, criticism given in the most mild and helpful form sends the author into a pit of despair, and then I just end up feeling like I'm a puppy-kicker. "I really liked your pacing; you've got a great grasp of how to keep a story moving! One thing you might want to consider, though, is whether or not your main character should have such an easy time overcoming all their obstacles." "Thank you for letting me know. You're right. I've burned the story, and am withdrawing from the group. I suck." Oi.
