The needs of a bright child who can't read are totally different to a child who has a low IQ and can't read are different. (Although both types of children equally deserve help.)
A profoundly dyslexic child can still learn other things to a high standard even if they cannot read. Placing a profoundly dyslexic child in a moderately learning difficulties school means the child does not get stretched at non reading activities. However if the same child is placed in a normal mainstream secondary school then they will struggle to cope with all the reading required.
Why are there not units/special schools for bright, but severely dyslexic kids?