"On the inability to learn, I tend to disagree. Every child can learn. They are certainly not able to learn the same things or learn at the same pace, but they can all learn. Severly disabled children can learn how to take care of themselves or how to interact and communicate."
Which merely reinforces my point; they can do this same thing without ever setting foot in a classroom.
Why promote them up through the grades when everyone realizes by the fifth grade that a kid who can't read is not going to magically come back in the fall a star pupil?
There are many situations to consider. Some LD kids spend a majority of the day in the regular classroom, some spend very little time there (lunch, art, PE). Some are educated in different schools, some are educated at home. There is quite a bit of evidence to support inclusion of LD or disabled kids in a normal classroom setting, as much as possible. It is beneficial for both the LD and non-LD kids. There are some kids that should never be in a regular classroom setting. I am not sure I understand what you are saying. Do you want to keep special ed. students out of regular classrooms? Do you want them in Special ed only classrooms? (this is much more expensive)