Yeah, I'm not saying it's an absolute necessity, but it's one more special skill that makes the lives of the people you are auditioning for, that much easier. And you also don't need to be a brilliant reader. I never took an extensive theory course and can't really play the piano. Sight-singing is a little different from being able to really read music. Sight singing is an easily learnable skill. I learned in high school, primarily from taking vocal scores out of the library and following along with the recordings of shows, to the music. You quickly lear about where the notes are going, their values, their dynamics and eventually start to understand what's in front of you in an entirely different light.
Most of all, you really need to know some basics. When a musical director says to you "add a crescendo at the end of the second system." You shouldn't have to be lost. And learning about it is not really time consuming or troublesome.
Again...it's a skill that will put you a few inches ahead of a performer who doesn't have that particular skill.
"There is nothing scary about life, if you are not attached to the results." - Conversations With God