Well, IMO, it puts everyone into a mental arms-race reminiscent of steroid use in bodybuilding competitions.
If you told me I was going to take a standardized test where only the top 10% make the cut, and everyone serious about winning is taking a mental PED, then yeah I am going to feel intrinsic pressure to step-up and also engage in that mental arms race. I would've been just as happy to not do that if I were given a more level playing field.
It's no secret that there is a war going on to commoditize attention. I want to live in a world where people hone their ability to push back against having their attention leeched not one where everyone raises their baseline to cope temporarily.
I also have to think that, long term, it's kind of disrespectful to those with ADHD. If expectations of attention rise due to uptake in mental PEDs then where does that leave those who were originally disadvantaged? They lose their ability to exist on a more level playing field.
Bit of a tangent, but many years ago when I was wrapping up college, I was in a Calc III class over summer. The course was being taught at an advanced pace and basically everyone in the class was struggling after the first exam. The professor addressed the class about the poor performance and then called out a student in the front row. The professor said, "You! You did well on the exam. What was your secret to studying?" and, not missing a beat, the student responded, "Adderall!" I took the hint, bought some from a friend, and suddenly found myself passing the class with flying colors. It felt good in the moment, but honestly only because I wasn't being graded on a curve. It would've been wildly demoralizing if I'd been in direct competition with my peers.
I know life isn't fair and it's a pipedream to clamber on about desiring a level playing field, but surely we can agree that physical fitness competitions are more interesting when they are separated between "natty" and "enhanced" competitors? And that if we eliminated that separation that those without PEDs are disadvantaged? I don't see society making that sort of separation for mental fitness and so I am concerned about letting stimulants loose on the population.