it works fine if you use the red dot correctly with both eyes open.
your eyes work using stereoscopic vision assuming you are right handed then you will look at your target with your right eye (usually strongest in right handers) and the left eye will pick up the image of the sight and your brain puts the 2 together but if you close one eye and look down the sight like you would a rifle scope you are correct you will see nothing but the back of the feed.....all red dot and reflex sights are supposed to be used with both eyes open.
assuming you don't have it pushed right up to the feed tube then they work, plenty of scenario players use dot sights....its all a personal choice thing but if you set it up right then the feed tube isn't going to be in the way.