...that we are simply pond slime made good, there is not much of a basis for 'rationality', in the end, is there?
From "slime" to more complex organisms and eventually ones cognizant enough to think more rationally, and maybe find themselves an "over-arching raison d'etre", but then again, do you necessarily need one of those to act rationally anyway?
I can't say that I've got one, or I haven't seen it around lately at least. But I do try to not do random stupid things regardless. Because that doesn't usually work out so well, which is reason enough in of itself. If I kick the cat he'll bite me, and that stings.
And similarly, Buddhist practice aims towards undoing the stupid habits we have that it turns out, aren't doing so well for us, and actually lead to more stress. I don't believe a philosophical justification is necessary to start engaging in that practice.
It is significant that in Buddhism, only the human being is able to realize Nibbana, is it not? Animals and beings in other realms, cannot. Not even the Gods can.
I believe the concept is that the human realm is best suited to realizing awakening since it's between the distracting extremes of suffering and pleasure. Though the gods/devas/whatevers can as well, if they practice properly. And additionally people who reach a certain intermediate degree of awakening are likely to be reborn in such and such deva realms, wherein they presumably finish the job eventually. Or so it goes. Point is, not just humans.