The argument is of course based on a faulty premise to begin with: that mechanical player-character effectiveness is what makes playing an RPG enjoyable. That way lies the utter, and now discredited, madness of 4th edition (which is linked to the equally mad and equally discredited character optimalisation mayhem that can be found on 3rd edition fora online); and even if you believe in that premise, you must recognise that it is simply an axiom and the argument is circular.
But be that as it may, I sometimes wonder if, even in purely mechanical terms, what we used to call "min-maxing" is genuinely advantageous. If your magic-user has an INT of 9 but a STR of 18, he can still cast spells, but he will also be very useful in combat. If your fighter has a STR of 9 but a WIS of 18, he will still be very useful in a fight but can also resist magic. And so on.
This applies more to some stats than others: Charisma is always useful irrespective of your class, whereas Intelligence is not particularly useful unless you are a magic-user. But by and large high stats are useful whatever you are, and provided you have at least a 9 in your prime requisite, you will be largely effective in doing the thing which your class is best at.
Is the notion of 'sub-optimal' choice just received wisdom that too few people question? Or do I simply misunderstand the argument?