Star nailed it for me - access to the national curriculum. Mainstream doesn't enable many, many children to adequately access the NC, but it does cost less! Babysitting is not teaching, segregation is not inclusion. Education leads to inclusion in productive adult society - at my son's natural level.
Sadly mainstream does not have as standard, educators with the training, experience & expertise to support many of our children. TA= babysitter as an educational model sucks big time, even it is cost effective.
If he's HAPPY shelf stacking I'll have no objections, but if his dream was to chase butterflies then I'll feel I failed him. What I'm not prepared to do is saddle him with a lifetime of dependency when he could, with support, carve out his own path. The comment about helping him to exploit his strengths as his disabilities will narrow his available options really resonated with me.
At the moment DS is happy and engaged in mainstream with extensive support, (it only took 3 years while he learned NOTHING and was subjected to intolerable levels of distress that will take years to heal). I however, cannot imagine him coping in the hustle & bustle of a London comprehensive, even with support.
No matter how much I wish it, wishing doesn't make something so & as his Mum I can't afford to exist in a state of denial, unlike the professionals who in 20 years time won't even remember his name. We'd all prefer it if our kids didn't have SEN.