I want to be all nasty and say something about wasting money and space... but really, I want to be her friend too.
Everyone in our family dreams of a communal hobby space. Something like a loft room that is big enough for the three of us to have a spot and put a TV on the wall. Somewhere that is outside of 'public space' within the house so we can leave our work out.
If we ever build again and can afford it, we are planning to include something like that but bedrooms as well! OK, that wins hands down.