Re: 3rd down back
You most likely broadcast your intentions of passing on third down when you insert a specific RB. A thoroughbred RB is not necessarily your best pass blocker, therefore you give up the advantage of your best runner with overuse on 3rd and long and lose the element of surprise if you insert a pass blocker on anything except 3rd and long.
Lots of different tactics and strategies to manage depending on what down and what type of play you want to run. You have a better chance of executing using your best players at their strengths. You may increase your odds of executing a running play on third and long with the gadget of having the pass blocker in, but your chances of running with a pass blocker on first down isn;t fooling many.