Good question... I was just about to start a thread and ask this same thing...
I just mounted an Aimpoint ML on the flat top of my AR/M4gery using Aimpount's Rail Grabber QD mount. I sighted the ARMS #40 sight and co-witnessed the Aimpoint to the top of the sight post...
Little issue: This was BEFORE I NOTICED THE M4 RISER IN THE BOX, TOO!!!
Everything seemed to work fine with the Aimpoint sitting lower on the reciever. I would just keep both eyes on target, and the front post seemed to fade away. The iron sights were in the upper 1/3 of the Aimpoint, but were still useable.
Now I've put the riser on... and get this: With no adjustment whatsoever, the Aimpoint's red dot seems to be in the same place as it was in the lower position... co-witnessed with the tip of the front sight post! Explain that!?!
I've got a little better field of view through the Aimpoint whey using the iron sights (as they are now in the lower 1/3 of the field of view), but otherwise, no noticable difference.
Is the added hight wort it?
Is there any benifit from having the sight plane closer/lower on the reciever, and closer to the plane of the barrel?
Any other benifits?