Puff pastry rises up (like a croissant) when baked because of the trapped air in the folds, plus it can be a bit greasy due to the extra butter. If you used puff pastry in the base of a quiche or flan, then it wouldn't rise much due to the weight of fillings above, but it would just be a greasy blob ! However, if you used puff pastry over the top of something, then you would have a light flaky pastry topping. I always use shortcrust pastry when I am making a dish that will have a lot of filling above it. I also do tend to bake it *blind* for 10 minutes or so, just to ensure that the filling (esp when the filling is liquid - egg etc) doesn't make the base soggy. The sweet/savoury distinction is definitely not the difference ! When making an apple pie, I often add a little brown sugar to the (shortcrust) pastry. I also never bother even trying to make puff pastry from scratch - far too much effort when you can so easily buy it.