Which merely illustrates what an incredible advantage the ability to completely remove pop divergence on a single state ethos would be. You have a maximum of three (although in this case, you would always want to pick two and have the innate one be fanatic save for roleplaying reasons, as with any option which offers a powerful advantage with absolutely no negative consequences.)
The problem here is that you have an optional trait the effect of which is not to grant a bonus or have a static effect the worth of which can be quantified and balanced, but rather which has the effect of removing a variable amount of disadvantage from a wholly different variable, a variable which also happens to be a core part of gameplay and a major limiting factor on some pretty important things, like the maximum size of an empire which can feasably hold together over the long term.
It has never been concretely determined that testosterone has any consistent impact on aggression in humans. Many studies report that it does, but a similar number find no correlation. This is not helped by the fact that it is very difficult both to accurately measure testosterone levels (in a living body) or to adequately define aggression.
Again, I think we have to beware of the notion of using animals as a template for how aliens might think or behave, because it probably shouldn't be any more believable than using humans as a template.
If we can't agree on genetics, basic bio chemistry or general lines of thought then this discussion is no longer possible to continue, so I won't debate any further on the science of the issue, since we simply are not compatible in our thinking process. Edit: I just think that if we really can't agree to understand what I meant in referencing testosterone (using it as a basic means of saying aggression causing hormone) in addition to my mentioning of physiology so obviously larger/more emphasized aggressive parts of the brain as we clearly see in some animals then we really can't continue can we? we both have our beliefs which we both feel are based on fact, I can find studies to support my beliefs, and I'm sure you can to, neither of us are experts so why bother?
As far as gameplay, they'd still have divergence on a single axiom at least (or on 2 if they put only one point in their innate) and as we've seen with the Blorg a single divergence is enough to cause problems, infact we haven't seen any divergence from xenophilia for them (at least none that caused political parties) yet they still had problems...why would preventing drift on a single axiom negate the problem entirely? you'd still have materialism versus sprituality, and militancy versus pacifism if you locked in the xenophilia as a trait.
Also not everything has to be 'optimal' who cares if it costs 2-3 trait points? for some of us it'd be worth it just to make the race we want, I highly doubt it'd be a huge advantage, as we've seen that the Blorg still had problems, and with this we'd be incapable of changing our government ethos, for example say we're xenophobic, but forming a federation would be really helpful, we can't shift to xenophilia and have better relations, instead we're stuck with our choice without gene engineering. It's not a big disadvantage, but then again it isn't a huge bonus either.
That being said, I'm done arguing this matter, so I'll concede the point to you for what it's worth, in the end it isn't worth the trouble of adding it, since we have seen ethos divergence is incredibly rare for fanatical ethos anyway, it's more of a fluff matter/tiny detail for some people than it is a big feature.