When you rotate the display the image quality drops significantly. The change in hue is very high on the vertical and you'll notice it even if you don't pay attention.
What you say is true if you rotate the cheaper TN based monitors. I have a Dell 24' monitor in portrait mode and it looks just fine as it is an IPS monitor.
I also have one Dell and one HP IPS monitor in portrait mode and they don't look fine at all. They are barely acceptable. There's no hue degradation, but there's a huge contrast degradation plus font subpixel rendering is broken.
I do see a difference between the portrait monitor and the others (it seats between two TN 24' Samsungs) but it is perfectably acceptable for writing code.
Cheaper TN panels are better at fast moving graphics (video games for example), but have a poor viewing angle. More expensive IPS panels have a better viewing angle, but not as good at moving graphics. I have a Dell IPS panel, which I specifically bought to use in portrait mode.