This is on a level with seat recline, endless arguments between two camps who both think their answer is correct and wont budge.

Personally, I'm with the it's my window seat (I probably paid to select it), it's my decision whether I want to watch/photograph the ground/sunset/stars going by or not. I'll close it (or otherwise block it somehow) if the sun is going to be shining directly in somebodies face, but apart from that, if it bothers you put on an eyeshade.

Its been covered before, a search should find many threads on it.