Understudies

I have just read the Phantom thread and started thinking about understudies. I have been one myself. Yes, sometimes people do "boo" when it is announced that the understudy will perform, for whatever reason. They have booked to see the name they know, or think they know, and are understandably disappointed. Flip side: they often receive a knock out performance from someone who knows they may never get to perform that part again and is therefore determined that they will excel. Sometimes, as an audience member, you may get to see an understudy perform who goes on to bigger and better things. For example, I remember seeing a matinee of Jesus Christ Superstar where the understudies performed...at that point they were Trevor White and Marcia Hines. We saw Jon English too though, and I have never forgotten any of them. I realise this shows my age! An understudy is often viewed as "potential future lead in training, just hasn't got the name/fame yet". Seen in that light there's nothing wrong in seeing them perform. I wonder about other people's thoughts? Do you feel cheated if the understudy performs? Or do you recognise the benefits of receiving that performance?