I don't know MabusAltarn's reasons, but I can provide you my own.
First of all, some of the AI personalities are already quite aggressive. This is primarily the case with AIP 8 (Drath, Krynn, Terrans, and Thalan) and, to some extent, the AIP 11 variant of the Arceans.
Secondly, the more aggressive a race is, the more it focuses on warfare. This is at the expense of everything else. The infrastructure of a highly aggressive race will be much weaker than that of a less aggressive one.
You can easily verify this by taking a look at the Yor and the Drengin. Both races use the same AI personality (7) as a base, but the Drengin are much more aggressive. In the game, the Yor will almost always have a better infrastructure, even if the Drengin have a better starting position.
If you are playing on a very high difficulty (like Masochistic, for example), then this won't be much of an issue. The bonuses, the AI receives, will offset most of the problems. However, it is noticeable on lower difficulties (like Tough and below).
In my mod, I'm currently using these values:
Drengin 70
Altarians 0
Arceans 60
Thalan 60
Korath 100
However, I'm considering to change them, because some of those races are still underperforming. The only exception are the Altarians. Their performance is pretty good, but, even with a aggression of 0, they still have very high tendencies to go on Holy Crusades, if there is evil afoot.