If you're stuck in the city anyway, and it's 30 degrees outside and worse inside, and you can't imagine one more sweaty stuffy night in that damn bedroom. If it's a Friday and you've been eating baguette & St Andre cheese & garlic paté in the park and drinking tall boys too but not quite ready for the adventure to end. And you happen to own an air mattress and a nice little Lord Fauntleroy lamp and you're both reading fantasy novels and the cat's feeling adventurous. If there's no rain in the forecast and the bugs aren't all that bad, if you're three stories above the world but still have a leafy canopy above your heads.
There's not much of anything better than balcony slumber parties.