Portrait isn't such a great option without IPS panels, due to the poor viewing angles on cheaper screens - especially from slightly above or below the screen when it is in landscape. This means that the side monitors are quite discoloured when you use them in a portrait position.
Also, many monitors seem to have thicker bezels on the top/bottom of the screen, which I imagine would be more jarring without bezel correction turned on, and hide relatively more of the scene with bezel correction enabled.