I agree with Dramamama that giving Verkaik an alternate can set a precedent for other actresses who come in after her. What's to happen if she does in fact get one and then the actress who comes in after her wants one as well?
I am well aware that there have been performers who have played the role who have taken a night or two off here and there to protect their voice. I do think that this role should have an alternate. Now, I know that what I'm about to say is apples and oranges but if Phantom can have one with Christine (who's hardest note to sing is pre recorded) then Wicked should have one as well. I also feel though that when an actress misses a show to protect her voice it's not uncommon for them to get bashed for not doing their job etc. I am not naming names out of refusal to do so, but only because I feel that this is something that happens frequently with whoever is playing the part.
"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