This is a good question Katie, and one that I have been asking myself while I have been reading the articles. When it comes to grouping students sometimes it is hard for teachers not to group by ability, especially if teacher has no assistant. It is easier for the teacher to get to work with groups on a given subject than to work with students one-on-one because of the amount of time in the classroom that can be spent each day on that subject. Then there are the facts that higher level groups don't progress and lower level groups worsen, for various reasons like teachers and students have low expectations, and the students resisting challenging work. I think groups are good for children because it teaches them to interact with others and develop social skills. As far as ability grouping though, I'm not so sure.
I know from working with two little boys I use to babysit, that children of different levels tend to get competitive, but in this situation it turned out to be a good thing. One boy was a third grader and the other was in kindergarten and the third grader was starting to learn multiplication. Every afternoon we would work on his tables and the kindergartener wanted to join in too. He ended up catching on quicker than the third grader, and this motivated him (the third grader) to do better! By the end of the year I had a third grader and a kindergartener that knew their multiplication tables. So in this instance, a diverse level pair turned out to be a learning experience for both involved!