There can be harmul side-effects and that topic is not covered by the article; on the other hand, pure evolutionary argument can be doubted because of changed environment.

If I stimulate my brain, it is natural to assume my brain requires more energy now. So I probably need more glucose. In evolutionary relevant context, that would make me more likely to starve - after all, I would need more highly valued energy and thinking clearly wouldn't make a killed bull magically appear before me.

This is still true for the most of the Earth's population. It is not true for many of LessWrong readers, though. There are some primarily-mental jobs now (in some places of the world - the places where LessWrong readers come from). Keeping more things in you mind means being a better programmer, teacher, scientific researcher. Being better at your profession often helps you to evade starvation. And getting needed amount of calories - if you already know where to get all these vitamins and microelements - is trivial in these parts of the world.

So, this modification was not a benefit earlier, and it was quite costly; both factors are significantly reduced in some parts of modern world.

Of course, increased mental capability can lead to some personality traits that make it harder to reproduce; but that is again a question of side-effects and not a self-evident thing. If you consider it harmful, you can try to spend effort on fighting these side-effects - some people report significant success..