Porches in the wrong place
It's nice to have a porch. It's even nicer when that porch is screened in so you can enjoy the outdoors without the mosquitoes and bees that are often part of the deal. So no one's debating the merits of a screened-in porch, but what can be a problem is where you put such a structure.
There Goes the Sun Where do most people put their porches? Right outside the living room. The problem is what then happens to the living room. Whatever breezes and light the room used to get have now vanished.
Inside Out Views from said living or family room are now ruined. Shade rules the roost, and looking out the windows means peering through a darkened space full of outdoor furniture you don't get to use (in most climes) several months out of the year.
Just Passing Through When you put a porch outside the living room, you turn what should be a destination into a passageway. Wall space is given over to the entryways to the porch, doors need proper clearance (eating into usable floor space), and the living room loses its definition.