Saveusmike, there are three possible actresses who can play the role of Elphaba in any given company of the show. The lead, the standby and the understudy. If for example, the lead is out sick and they put the standby on for one performance, then they are down to two possible actresses in the theatre who can play the role. If they have the standby become the alternate then there is no difference between that and having the lead being out sick and having the alternate go on for one performance. If there is a problem with the alternate on the day that they are supposed to go on then there is the understudy who can do it.
I also disagree with you that the role of Christine is one that warrants an understudy. There is one very high note that she has to sing which if it were done live 8 times a week could be a problem. But, that is in the title song and we all know that that song is pre recorded. There have been a few actresses who have played the role who have said that they feel that they could have done the show 8 times a week with out the alternate. The role, although operatically sung, is not that hard to sing. Keep in mind that there are a vast majority of actresses who have played the role of Christine who have had a fair amount of training in opera.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll