Is energy what kills deer? If so, how much energy does it take to kill a deer? And, after you reach that point, what is the additional benefit of having more energy?
(and regardless of the above, hasn't it been irrefutably proven by now that a 6.5MB is perfectly capable of reliably killing deer at 600 yards? If that is true, what is the benefit of the bigger cartridge toward the goal of killing that deer? If the exact same shot placement with a 30-06 inside 600 yards will result in recovering a deer that would be lost with a 6.5MB I want to hear that--THAT is the only thing that's relevant to the original question as far as I can see)
Personally, I have always been drawn toward the approach of using just barely enough of a tool to confidently and reliably accomplish my task. The "sledgehammer to drive a finishing nail" approach was never my style, and I am fortunate enough to own multiple rifles to suit a range of shooting requirements. I own a 6.5mb, but its a range rifle for the most part, its my 7mm08 that I reach for most times instead of my 30-06, for the same reason--the smaller gun is plenty, and I'd rather minimize meat damage and carry the gun that's more pleasant to tote and shoot. This conversation just gets kind of comical going around in circles. It seems until there is a strong argument for the smaller caliber NOT being sufficient, that there is realistically no universal definition of "better".