You’re right. The brain needs to dump useless stuff...
I think it is something like creativity -vs- autism. Most people are not flaky crazy creative, and neither are they rigidly autistic - they are somewhere in the middle.
Now a gene variation that makes you more mathematically inclined and intellectually rigorous may also make you more prone to autism, while loosening up that rigidity can lead to more free wheeling creativity, but can also lead to craziness of a different sort.
If this gene was evolutionarily conserved from the time when mice and men shared a common ancestor, it is obviously doing something worthwhile.
My father, an engineer, said “When all you have is a hammer, problems look like nails.”
I suppose to a guy who runs mice through mazes, a mouse who can memorize a maze better seems “smarter”.
But there is a lot more to it than THAT!
I would like to see how these knockout mice (mice without the gene that helps them to forget, memory mice lets call them) would survive and/or interact in a situation that more closely replicates the wild.
I would bet that they perform quite a bit less well in “life”, than in the maze.