What do you envision when someone says "homeschool co-op?" A study group. An activity club. In practice, it could look like either. A homeschool co-op which suits its members will certainly benefit them. On the other hand, I can imagine some scenarios which would not serve those involved. However, let me focus on what works.
My children and I regularly participate in activities with an all-inclusive homeschool group with a significant unschooling membership. That means we have both formal and informal activities from which to choose. The group hosts several clubs, i.e, LEGO club, Spanish club, Inventor's club, as well as special events (art show, science fair), park days and parties, and fun stuff for the parents, too.
Why do I like the group so much?
It's all-inclusive by definition and in practice. Members need not fit themselves to the group. Although, members span the homeschooling spectrum from formal school-at-home to radical unschooling, I've never seen an incident of judgment. The kids all play and learn together without any knowledge of the details of one another's homeschool differences.
The group is big. It has hundreds of member families. My children have access to a diverse community of families who homeschool, too.
What would ruin our group?
If the group imposed a value system at odds with my family's value system, we would not enjoy the group.
If the group imposed a particular homeschooling philosophy, such as strictly structured, we would not enjoy the group.
In other words, the diversity and respect for diversity that exists in our group is the key to its success. We all get more out of the group for its diversity. The homeschooling movement at large could benefit from this perspective.
What benefits do the children get from the group?
They meet a lot of people and develop friendships with people who participate in the same clubs. They learn from the expertise of the parents, i.e., Spanish from a native speaker, science from a technology professional, robotics from an engineer.
The kids get to play for extended periods of time during regular play dates and parties.
They get to share their enthusiasm for individual interests. My daughter readily tells everything she can think of about her passion for marine life.
The kids practice their social skills in a natural way. They learn about emotions, manners, etc., within the context of growing friendships.
Homeschool co-ops can significantly benefit homeschoolers. Not because they don't otherwise get out into the world, because they do, but because the co-op brings homeschoolers together.