I was in contact with 3 or 4 of them in our area when we were thinking about continuing my son's homeschooling into the high school years.
I never saw the "hubris" you mention - I never met a parent who was reluctant to have an expert in the more difficult subjects. In fact, you would be amazed at the qualifications of some of the folks in these co-ops. Just one example -- a retired Army Colonel of Engineers with a doctorate in electrical engineering was teaching the kids calculus, along with the principles of navigation and applying math to "real world" problems like moving dirt and building amplifiers . . . and the kids were having a ball!
What he wasn't doing was trying to influence the kids politically or socially . . . which is what many homeschool parents are trying to avoid. It's not just the lack of competence that annoys parents, it's the ideological subtext that's injected into so many classrooms. To paraphrase, "shut up and teach!"
And it may be hubristic of me, but I would put my academic qualifications up against those of any public high school teacher. I'm not qualified to teach math or science at the high school level, but I could teach English, English lit. or history at college level, and if I had a chance to brush up I could teach up to intermediate level German or beginning Latin.
So, how are the kids you homeschool going to do on the following standard high school math question:
Two automobiles start out in opposite directions. Each go 40 miles; make 90-degree left turns and go 30 miles. How far apart are they?
Bump...and I would add:
My favorite thing about homeschooling is learning with my children. I don't have a "science degree" but I CAN read...it's certainly not difficult to open a Biology book, hike in the woods for fungi, collect insects, or look at things under our microscope, etc. Why would one need to be a professional to explore and encourage a passion for the sciences?