So I read an article that talked about ability grouping (A Classroom Divided), and I think it is called grouping now because "tracking" has such negative connotations. I do believe teachers should create differentiatend lessons to reach all students, make all students reach their potential - without using time as major factor. But ability grouping needs to be monitored closely because we have a fine line between differentiation and true "tracking."
Tracking, to me, is when a student is expected to do less, ends up in lower classes and simply pushed to HS graduation with no thought of the real skills he/she will need to make a good living after HS (fifty years ago - they were our blue collar segment - which could make a good living). And lets be clear - not everyone needs college - but everyone needs to be able to do problem solving and higher order thinking. And those things are the first to go in a lower expectation class, those things are hardest to teach. Problem solving and higher order thinking are what gives people the ability to roll with our ever-changing world.
Differentiation, in my opinion with respect to ability grouping, is expecting the same problem solving and higher order thinking but delivering it thru a variety of ways (and not slower & louder). Thus setting up your classroom in a multiple number of ways - where one way is ability groupings. BUT, if that is the only way the students are divided then you are on a very slippery slope!
Once you only use ability grouping I think you are setting up a dynamic that will not be good for the lower students. Every class has projects that can use heterogeneous ability groups, and thus can make sure we are putting students in multiple situations for learning. I know the argument is the "smarter" students do all the work, but isn't it our job to make sure that doesn't happen. (Or that the "smarter" students get everyone to learn it too?) The real problem is the students who struggle are not challenged to get the same set of skills.
But as long as education is set by age we will have these trappings.