I'd say that the best qualification they have is that they know their kids, and care deeply about them. They provide student/teacher ratios that are impossible to match.

Each child has different needs and different strengths and weaknesses. They progress in each subject at different paces. Our daughter is 5th grade age. At this point, we're still comfortable with the education we can give her ourselves (using materials created by people with a good deal of experience teaching their subject areas!). There will likely be times when she has reached the point where we can no longer adequately teach her certain subjects. But, there are tons of resources today and more coming every day for teaching those subjects.

I was talking with one of the other dads in our group last week and they're deciding how to proceed with their 8th grader for next year, when she'd be entering high school. One of the options they're exploring is one in which she would be able to attend courses at a local community college (for free via their local school district). Programs like that exist in many places.

Homeschooling families don't live in a vacuum.