String,
The thing is with specialised schools - as with the bipartitite selective system - is that they don't acknowledge either the variation within a specific SEN diagnosis, nor the fact that a child can have multiple SENs (or a mix of acute SEN and very high ability in one or a number of areas).
I am going to use the example of visual impairment, just for the sake of argument, though the examples below would also apply to any other specific SEN.
Within visual impairment, there is a huge range of 'ability' - from mild short sight or astigmatism, through colour blindness, acute short sight, partial sightedness of different degrees, all the way through to total blindness. Where should the line be drawn for 'specialist schooling? Partial sightedness? Total blindness? Somewhere in between? If theer is going to be an 'absolute standard' for admission to specialist schooling, there will be children of almost identical sight on either side of the cut-off, but one would stay in mainstream and the other would be sent to a specialist school.
Also, consider the other needs of the child. If a child is blind AND deaf, do they attend the specialist school for the blind, or the one for the deaf? If they are blind and autistic, or blind and cognitively impaired, or blind and have a physical disability or cerebral palsy, which SEN 'wins' in terms of which specialist school they attend? Or is there a non-specialist 'school fro all children with all SEN', in which case how is that better than mainstream? Or alternatively, is there a proliferation of tiny acutely specialised schools - one for the deaf, one for the blind, one for the deaf blind, one for the autistic blind - to which children have to travel huge distances?
In addition, why should the child be defined by their disability rather than their ability? What about a child who is blind but Oxbridge material? Blind but musically gifted? Should they be educated as 'blind' or as 'able'? Will a specialist school for the blind have the facilities to teach a very able child as well as the neighbouring mainstream school with much larger numbers? Would they be as able to give the musically gifted child the education they needed as a music school?
As I say, this argument applies across the board, to all SEN. Yes, co-location of some specialist provision with mainstream schools may well be a sensible way of dealing with children with very severe SEN. However saying that 'any child with any SEN should expect to attend a specialist school' is nonsense.