Just because Donna's number of absenses are publicly known, you think she is hurting the show. Find out how many times other big Broadway stars have been out of the show and then I might agree with you.
That really doesn't sound that bad. I don't know about Donna, but I know that Kristin knew she'd be missing the show a lot when she signed on. She told Stephen Schwartz she couldn't commit to such a large Broadway project, but he got her to look at the script, and she decided to do it. She knew she had films and concerts, the producers decided that was ok, so stop trying to say she's horrible for missing. Something tells me that Donna also has legit excuses: that's the way things work.