I think it is great that there are both point buy and dice rolls for ability rolls in the character creation. Let people use what they find the most fun.
It is clear that if you roll your character's abilities you can easily reroll into very powerful characters with several abilities above 16, even multiple 18s without waiting too long.
A motivation to play point roll characters could be getting an achievement if completing the game using only point roll created characters. Just a thought.
Dice roll characters if you want to be the heroes and point roll characters for a harder go at being heroes. ;)
I get what you are saying, but I am not personally a fan of achievements. Mostly because I find them to be meaningless at best, but also because there will always be a mod/cheat that gives them all to you anyway. I would prefer that any time and energy the developer might spend on even considering achievements, went to improving an existing function or implementing something new that affects gameplay instead.
Aside from that, dice rolled stats are not necessarily better than point bought. Point buy is a guaranteed pool. I have had P&P campaigns where you take your initial dice rolls and live with them. Many players enjoy playing purposefully handicapped characters as well. Shouldn't I get an achievement for finishing the campaign with an all 10's character that I free rolled?