Performing on bway is just like any other job. Actors don't/can't just take off because its a two show day. They only have a certain number of sick days -- just like in the real world. There are a few roles that officially have an alternate -- meaning that it is a permanent schedule for someone else to perform. (Currently, I believe only Evita and Phantom have alternate actressess.)
Actors get vacation time and of course, they do get sick -- but it really isn't that often that folks are out. (There are always exceptions, of course.)
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.