Bigbulls,
Ofcourse, you are correct in that explanation of how .006 might actually be 12 thousandths of an inch from one arrow to the next......and a .003 might actually be .006 variance from one arrow in a group to the next. However, as has often been pointed out in the ICS carbon debate threads, not all of the arrows even meet those advertised specs initially and then they tend to degrade over time with extended usage so a .006 difference in arrow straightness is probably the least of someone's concerns when considering two different carbon grade shafts.
I have no doubt that you shoot better with the slightly straighter carbons but I do believe that this is not necessarily the case with the majority of shooters out there. One also needs to consider the specific brand, and the subsequent quality control of that brand, when making references in this discussion. Some brands, such as Goldtip, might see a large difference between their low end and mid grade to high end shafts where others, though they advertise a sizeable difference, are not really that different in terms of consistancy for straightness, spine or weight from one "grade" shaft to the next.
Then add a fixed blade head on the end and there was no consistancy at all with them
I cannot argue that point but I have found, more often than not, that the cut of the arrow and the subsequent insert alignment is more often at fault than the straightness of the shaft....again, at least in what I run into regularly.
I am not trying to be argumentative but rather just offering a different point of view on the subject.