I'm not in the industry. I was in two musicals in high school and danced locally for several years when I was younger but that is as far as it goes. From an "outsider's" perspective, or simply as a member of the audience, I would rather see an understudy than a sickly performer who is doing a less-than-standard job.
Not too mention that, as many others have already said, being sick can basically suck the life out of you. I think it is unfair to expect the performers to go on when they are vomitting or running high fevers, etc. I work in the food service industry and am appalled at how often the management expects employees to come to work ill, despite the fact that we are serving food to the public and highly contagious!
I've also worked in corporate style offices and can say the same thing. Everyone is expected to come to work sick, but I'd imagine it would be easier to do an office job while sick as oppoesed to being onstage performing for thousands of people.
I once saw a show where the lead was out sick and so was the understudy, so they pulled another understudy from an out of town performance (and I am not making this up!) If that starts happening in SA then I think it is time to worry, but for now, they have the swings, so why not use them?
"Thought is suspect, and money is their idol, and nothing is okay unless it's scripted in their Bible." -Spring Awakening