So I am a little confused. Are you saying you would use a larger caliber to make up for poor or less than ideal shot placement?
A 6.5 in the kill zone will kill an elk just fine according the many kills acquired on this site alone. I know several guys who never post on here that do well with 6.5's. To me the question is if you can shoot effectively at 800 yards, lets say, then caliber consideration is purely for the effectiveness of the bullet based on mass and velocity. A heart shot elk with a 140 or a 250 gr bullet is just as dead, right.
Although I agree that larger bullets at greater speeds are more ideal for larger game I would not shoot at a range I am not comfortable with regardless. I would imagine most people would agree, they would do the same.
So it boils down to the age old idea that shot placement trumps everything within a certain amount of reason. American hunters have been set on the big 30's for so long that we tend to ignore the fact that a poorly placed bullet is just that, poor. In the event that occurs we tend to fall back on bigger is better, simply because more damage can occur, and therefore better chances of recovery results. What some people find is that "they" shoot the 6.5's better than they do the larger pills and wind up with better overall precision, plus they know the shot placement counts more and slop is not as acceptable.
I might be wrong, but this how I tend to think about the arguments. They will all do the job if the job is done right.