Cats will urinate outside the litter box for a lot of reasons. But when they have normal stool outside of the box, it's usually a behavioral issue. Especially if it's, say, on your couch.
So here's the situation. Client comes in with a cat who is actually urinating and defecting outside the box. Although I can't rule out a medical component I suggested there were some behavioral components. Now the recommended amount of litter boxes to have is one for every cat plus one. While some cats are not nearly so private or picky, some are. Since this family had one litter box, which they admitted to only cleaning a few times a week, and two cats, I suggested they add a few more boxes and clean them (gasp) daily. Their response was "you must be crazy."
Seriously? Who is the one with poop on their couch?
Really, if someone asked you to share a walk-in toilet with another person and it was only cleaned two times a week how happy would you be? You would probably poop on the couch too.