I had just a morph scan with Natty. With Phoebe I had a 'dating' scan and the morph. If I were to have another kid I'd just go with the morph scan.
Even though I'm into natural birth and have aimed for as natural a birth as I could manage both times, I still think a level of medical care is part of responsible parenting. I am not comfortable with free birth, and I don't think I'd be comfortable without the morph scan.
I agree in that I don't think anything can prepare you to look after a kid with special needs. However I would want to have that scan in order to find out if my kid had an issue that'd require surgery. At both my morph scans we have had the heart looked at very closely as our family has a history of congenital heart defects.
So while I personally don't feel the scan is necessary to prepare me for any physical or mental disabilities (as I just don't think I could prepare myself for that eventuality) I would want to know if my baby would require urgent surgical intervention because we have increased likelihood of that and I'd want it without delay.