Oh goody, here we go again. I thought it had been a while since that was asked.
For the same reason you need Police, Fire and Paramedics. They all do different things, have different specialisations and closing the Paramedics down and giving them shiny yellow hats doesn't mean the requirement hasn't gone away.
I think that just about sums it up whilst removing inter-service parochialism and offering a level of sophistication in the analysis exceeding that in the question.
Last edited by Melchett01; 14th Nov 2017 at 18:38.