Eliza isn't a terribly hard role to sing, but as others point out, she's on stage for virtually the entire show. Given that Ambrose has never sustained such a long run in a musical I'd say her overall attendance is pretty stellar. And if having an alternate one show a week allows her to continue for the remainder of the run missing little-to-no other performances, it's probably a good decision in the long run.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body