I never carried the M24A2, only the M24. I only know that NADS mount from previous research and from what I remember it wasn't in the system very long before better clip on systems came along. When I carried the M24 we had the Leupold M3A and PVS-10, I've never seen a M24 with the 6X optic on it, only a 10X.
In regards to the shift from fixed power optics to variable power optics I don't think it's a case of wanting a variable but needing a variable power optic for the environments that we are in today. Way back when doctrine typically told us that the sniper team was a two man team that was going out to collect intelligence and engage selected targets with precision rifle fire. In many ways that was the Cold War thinking, two guys sneaking about, taking a limited number of shots from a position, and moving on. For that type of operation the fixed 10X optics from Unertl and Leupold made sense, they were tough, reliable, had enough magnification to engage targets at distance but not so much magnification that the field of view was horrible or have mirage ruin the image quality. Skip ahead to the Global War on Terror and the old doctrine damn near gets turned on its head. As we were operating in both urban environments and mountainous terrainthe 10X optics was left wanting in more ways than one because it was too little magnification for those shots to the next ridge in Afghanistan and almost too much magnification for engaging multiple targets in those urban areas. Variable power optics provide shooters the flexibility to better see details at long range, dial it down for wider field of view in closer range engagements, and dial down the mag for use with clip on night sights. Variable power construction has also gotten a lot better and a lot more durable.