I used to play WoW years ago and sometimes game lead designer - Ghostcrawler would sometimes share his decision making process. One thing that stuck with me was him justifying removing some gameplay element, it went something like this (I am heavily paraphrasing, it's been like a decade) - players tend to do what gives optimum results even at expense of fun, even if it's a small advantage and to get it is tedious. It is better game design to remove that option so they can do the fun part without feeling guilty about not maximizing their perfomance.
If you add option to roll for you character and an option of fixed nr of stat points, the optimum gameplay solution is to keep rolling until you have much greater number of stat points when the "fixed" option would get you. Not only many players will spend great amount of time rolling until they get their "perfect" character it also makes the character more boring - with considerably less decision making of what strengths and weaknesses the character should have.
Setting aside pseudo moralising "oh it's their own fault if they have ruined their fun for them" having fixed nr of starting stats is much healthier option for the game, in my opinion, but as I said before, I am pretty sure they will go for the roll option.
