Dot sights are NOT like conventional optics. You don't close one eye, and you don't want it mounted to the rear, close to your eye, because you don't want your field of view limited by your view through the optic. These devices are designed to be mounted forward, and used with both eyes open, so that you see the dot as part of your entire view. This allows you to be VERY fast getting on target, and prevents you from losing situational awareness.
If you've only used magnified optics, this can take some getting used to. But mounting your optic to the rear will force you to "tunnel" by making you focus only on what you see through the tube/screen, and it will take longer to find additional targets, and you'll have trouble keeping track of what else is happening around you.
Some folks *do* have eyesight or other issues that require less-than-optimal setups, and you can tell by many pics of our military using them that they are both limited by their mounts, and in most cases, don't understand the benefits of forward-mounting either. And with M4s that are using a top-mounted PAQ-4 or PEQ-2, you can only have your optic so far forward before you'd run into your laser.
-Troy