Coming from a shoes-on culture I can give a couple points.
First, in the rare cases you describe (when was the last time you stepped on feces?) one would indeed leave their shoes outside. But in the day-to-day your shoes have some dust on them and that's it - you won't even notice it's there when compared to the dust that already accumulates daily. If you live in a climate where this assumption doesn't hold (snowy areas come to mind) then the equation changes.
Second, the floor is usually stone and therefore designed for being walked on with shoes. I would feel awkward walking on some polished wood, but if the difference between inside and outside is "this one is less coarse but just as hard" then I wouldn't see a good reason for taking my shoes off. I would probably avoid the rug, but that's me.
As for comfort, keeping your shoes on also means one less thing to do when going in and out. Do not underestimate the path with less friction.
First, in the rare cases you describe (when was the last time you stepped on feces?) one would indeed leave their shoes outside. But in the day-to-day your shoes have some dust on them and that's it - you won't even notice it's there when compared to the dust that already accumulates daily. If you live in a climate where this assumption doesn't hold (snowy areas come to mind) then the equation changes.
Second, the floor is usually stone and therefore designed for being walked on with shoes. I would feel awkward walking on some polished wood, but if the difference between inside and outside is "this one is less coarse but just as hard" then I wouldn't see a good reason for taking my shoes off. I would probably avoid the rug, but that's me.
As for comfort, keeping your shoes on also means one less thing to do when going in and out. Do not underestimate the path with less friction.