It's a little weird to have to detach myself - I can't really see the badness of their acting, even knowing that for this time, it's like having a party when the parents aren't home (Lady Bertram and Mrs. Norris not being parents.) I can't remember how old everyone else is, but Fanny, Julia, and Maria are in their late teens and very early twenties, roughly. Well, and Edmund is seven years older than Fanny. Tom and Edmund's argument is reminiscent of high school-centered tv shows I've seen. I suppose Mansfield Park can be compared to that kind of sitcom, and the comparison might be especially accurate for readers who disliked Edmund and Fanny and the treatment of morality in the novel. Of course, while it isn't covered in this chapter, Fanny's objections will include not wanting Henry Crawford behaving more inappropriately than he has already and not wanting her cousins to get hurt by him.
It also brings up the notion of class again - acting is to be appreciated, as long as the right people do it. It's that thing lesser people do that makes them interesting and entertaining. I don't think this means everyone except Edmund and Fanny are less snobbish though - it seems they want to do it because the 'not really forbidden but not quite appropriate' tinge makes it adventurous, not because they have any feeling for the lower classes and their culture and labor.
They also involve their romantic entanglements: Julia supports Maria not acting, Maria turns her engagement into a reason for her being more appropriate than Julia, and Edmund gives in because Mary Crawford said so. I'm pretty sure that getting the chance to flirt in a slightly different and suggestive situation is at least a bonus for Henry.
*I apparently love commas.