Effectively deployed TA's, where their area of expertise and training is taken into consideration, can make a significant difference to a child's academic and social/emotional progress. Poorly deployed TA's can hinder a child.
Equally, just because a teacher is 'trained', it does not necessarily mean that they are good at their job (or all elements of their job) or able to reach out and meet the needs of ALL children in their class. As with all professions - you get good ones and bad ones! Some teachers deliver fun, exciting and creative lessons (yet are disorganised, flighty and impulsive). Others deliver old school (classical conditioning) lessons, but are well organised, consistent and reliable. Some teachers have a fantastic grasp of BESD and a great empathy with children, others have exceptional subject knowledge but fall to pieces when faced with a BESD child.
On a similar note, I can think of a teacher within my setting who have been teaching for a very long time. When they began their teaching career Autism was virtually unheard of. During their long career (they are close to retirement) they have not received CPD regarding Autism (or if they have, they've remained very closed minded about it). Therefore, when faced with Autistic children their approach, manner and lack of understanding leads to them having a detrimental impact on that child. Through the TA being there, the TA engages the child, differentiates the learning and helps them remain in class (and mainstream education). On the other-hand, that teacher is loved by the majority of his class, he effectively facilitates their learning and their progress reflects that.
Over my time as a TA I have undertaken numerous CPD courses. Subsequently, in some areas I have become the 'more knowledgeable other' compared to the teacher. As an example, I worked with a year 6 teacher, and we had a number of children (five or six) who had significant gaps in their knowledge with regards to phonics. The teacher had little experience or interest in teaching phonics. Yet, during my time at the school I had taken part in numerous phonics training sessions. In this respect, it does not make sense for the teacher to deliver the learning for these children, when I was the more knowledgeable other. In this case, I would offer a short phonics intervention, a few mornings a week, where the long term planning was overseen by a specialist phonics teacher and the short term planning was created by me. The programme was structured and I was trained to deliver it.
However, put me in a situation where I had to deliver history or PE then I would be out of my comfort zone and it would not be benefiting the children. I am very aware of my limitations and professionally I am not afraid to admit them.