"The difference purely is the terms & conditions here will never allow you to return your ticket purely because an understudy is on."
If you buy a blue shirt from Marks & Spencer over the internet and go to pick it up at one of their stores and they give you a red shirt, do you just take the red shirt and say "oh well, stiff upper lip and all that"?
People pay to see Emma Stone. Her understudy may be excellent, her understudy may be even better than she is, but people pay for what they want and it's not to see the understudy.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.