I've been backstage at Mamma Mia oodles of times ummm Wicked, Aida, The Woman in White, Dirty Rotten Scoundrels, Legally Blonde, Spamalot, The Wedding Singer, Chicago, Fiddler on the Roof, La Cage aux Folles those the ones I have from the top of my head.
I've never done it and I don't plan on sitting on stage. My biggest problem with stage seating is I think it ruins the whole illusion that what you are seeing is real. As soon as someone sitting on stage moves you are distracted and are reminded that what you are seeing is just a show. If that even makes sense. Thoughts?