While I can make a case for durability, I can also make a case for aesthetics. Where aesthetics may have come into play is with regard to trade.
There was a saying that the Indians would trade all of their land for a few red feathers, to wear in their hair. Europeans who had early contact with the New Worlders recognized this intense sense of vanity, and soon tapped into it.
When one looks at how lithics were traded out from certain areas, what can be seen is that they were bartered as a sort of currency. And, with this in mind, it is not hard to see how views that pertained to aesthetics could have come into play.
In other words, if I were to make ten of my own heads to shoot, I might do just fine with ugly clunkers. But, if I make ten heads to trade, and ten other guys make ten heads to trade, then I may be able to do better, and get more bang for my heads, if they look better than the other ten guy's heads, given the buyers (traders) sense of vanity.
If this theory is correct, then it would probably be more noticeable in items that were likely vanity items. For example, if the Fulton Turkey Tail knife was worn on a cord, strung about the neck, it would have been a highly visible item, that everyone would have marveled at. So, in appealing to the buyers sense of vanity the maker could have used the best stone, the best flaking, the best of everything. And, once people starting obtaining such an item, there would have been some sense of vanity that would have compelled people to want to wear the best looking knife "on the block". And, everyone would have wanted one.
So, I can see lithic design being driven by both vanity, and by utilitarian purpose, depending on the context.