Without an education degree, every credit that would have been an education class (and often it's 30+ credits) gets replaced with a credit in a real course. That can only help make the teacher smarter and more educated. Only somebody with an education degree thinks you need an education degree to be a teacher.
The best teachers in life sometimes do not have the credentials for even becoming a teacher. You can learn just as much important information about life, liberty, and the pursuit of happiness just by talking to the homeless guy down the street. A degree doesn't make you a good teacher.
Yes, people can be highly intelligent and learn things without having a degree. Also, some people are naturally good at being teachers, or have learned the skills to teach well without a degree. Parents teach children all the time without a degree and do well at it. It should be the standard to have a degree to teach, but substitutes don't require one, and people shouldn't always be ruled out because they lack one.
A teacher without an educational degree is similar to a child's parent. Many parents are do not have educational degrees, yet are able to help their children. Therefore, a teacher without an educational degree is fairly equivalent to a parent. A teacher without an educational degree may have more life and professional experience that enhances their teaching ability.
While I believe an education degree would give a teacher tools and skills that put them above someone who does not have one I see no reason why someone who doesn't have an educational degree couldn't be an effective teacher, I believe that if that individual is intelligent enough he/she can be a very good teacher.
Teachers have to have degrees in the United States, so asking if a teacher can be effective without an educational degree is pointless. Without a degree, you're not a teacher. I'm sure there are people out there who can teach concepts to kids and they don't have degrees, but that doesn't make them a teacher as far as academia is concerned.