I think that his point was an AR, unlike most bolt rifles, typically requires you to raise the scope to get it inline with your eye when you have a proper cheek weld. There is no drop at the comb on an AR unlike a more traditional rifle. That drop at the comb often means the the cheekpiece on the stock isn't high enough for a proper cheek weld with the scope mounted as low as possible.
The only reasons that I could think of would be to mount a scope on the carry handle...I'd get a flat-top upper before messing with that...or mounting a HUGE optic like NV or thermal imaging optics that sit even higher than is normal for an AR.