I am not sure where you are getting the notion that there is a plethora of paras in schools, but there are definitely more now than there were when I started teaching. However, they are certainly needed, especially when the law states that all children must be educated in the least restrictive environment possible.
My last year of teaching (2 years ago), I had 22 students in my fifth grade class. One student was autistic, and he was on the lower functioning end of the spectrum...very challenging. Another student was bipolar and oppositional defiant, and he regularly threw things across the room, was violent toward both students and adults, and ran out of the classroom every time he was expected to do work. I had one para for a limited time each day. But hey, if my students (including the two mentioned above) didn't pass the state assessments, guess who got the blame?