Necroposting, but fuck it, I feel like arguing.
Because I think there's an argument to be made for their race being more than just stock reasons to supply plot. If you
wanted to do a critical read of the Simii, there's a few points you could connect for some depth.
1. The idea of them just being hyper-advanced earth-monkeys rather than an alien race is a pretty stock sci-fi trope. In fact, it's one of the oldest sci-fi tropes,
going all the way back to the 1860's, but
still used 150 years later.
2. The initial monkey tests of the 1960's were meant to test the rigors and stress of space on near-human biology. It's a pretty easy step to go from there to "uplifted monkeys meant to test the rigors of space and mutagens."
3. If you want to progress the idea a little further and a little darker, a near-human species with resistance to alien mutagens would make for a perfect colonizing/guinea-pig race for new and unknown planets and environments. Assuming that early humans saw them as having less worth than humans (since they're monkeys AND artificial), it would kind of make the Simii the perfect expendable species for setting up bases in hostile environments for humans to inhabit. (But that's a "dark humanity" read on things. I blame all the Halo novels I consume.)
4. To take the theory a little further, it would be entirely conceivable that the Simii's initial culture (whenever they established themselves as an independent state) would have been heavily influenced by humanity. If they were indeed created as a lab-rat species, they could very much have internalized the structures of bias and worth from their oppressors.
Note that advanced apes inheriting humanity's worst qualities is, again, a pretty standard scifi trope. So the idea of the simii having archaic views on gender identity and building xenophobic communities fits the trope pretty well.
But I guess this doesn't really make the argument that the simii fit TiTS, but rather that they fit sci-fi in general. If anything, I'm arguing that their traits aren't two-dimensional plot-props, regardless of the original author's purpose, but that there's some precedence for the concept as a whole in science fiction.
As far as having a TF-resistant race in a game about transformation, I wouldn't really know how to argue against that claim. Or if I'd even have a leg to stand on in that argument. Especially since I'm the one that wrote
unfuckable aliens into a game about fucking.