Well I actually like Dawn as a character, so perhaps my opinion will not be listened to by some, but I think Dawn had every right to do what she did. She did it for the good of the potentials, for the good of herself and especially for the good of Buffy. The house would have been chaos if Buffy stayed, disagreeing with every order. Nothing else could have happened, really.
The entire event is necessary for ALL of the characters - for Faith to grow as a slayer and leader, for the potentials, for Buffy to realise a) she does love/whatever Spike and b) she can take on Caleb.
And it was Dawn's house too. She had to make the decision. The entire season was based around Buffy making the hard choices and decisions, always ending up with her self-righteousness (not that you can blame her, but still...). It needed to be someone else to make that particular hard choice.
That's what I reckon, anyway.