teacher's unions are one of the few tools educators have to fight back against the chronic cuts to public education that have been occurring in America. this benefits students and educational facilities by helping get more state funding, helps keeps teacher pay at a more competitive level (hopefully keeping "good teachers" in their jobs), among other things

I think the points you raise are valid, but given what an abysmal state public education and unions in general are in America - I think the benefits of teacher unions greatly outweigh the costs