I think it's quite rude to keep the shade open when it's supposed to be a "sleeping" flight or long enough when people do want to sleep. Just as rude to open and close constantly throughout the flight if it's bright outside. Every time the shade is opened, it bathes the entire cabin in light and it can, as mentioned above, be jarring and wake people up.

There is often nothing to see. If you want to look go to the galley where there is often a window on the exit door. This is akin to using headphones vs speakers when listening to music. You may control the window shade but you also control your level of courtesy and consideration to others.