One of the selling points of the EOTech Model 512 when I first started hawking them was the fact that they used the common AA-size batteries rather than the esoteric N-cell used by the Model 511. When I purchased one of my own, I was happy with the fact that I could pick up batteries at any old convenience store to keep my optic running.
Just a few short years later, almost nothing I own runs on AA batteries except for my camera and the flashlight I keep in the bedroom for blackouts. All my other lights, from the light on my carbine to my Surefire "tactical" flashlight and even the little TwinTask 1L I keep in my purse for general flashlight chores, use lithium CR123 cells. This means I need to keep the little battery compartments in my foregrip and pistol grip filled with two different kinds of batteries. This is a pain in the butt. I need to get rich so I can replace my 512 with a 553 that takes the newer batteries. Plus it comes in flat dark earth, which would match my gun, and that's important to the style-conscious armchair commando like myself.