> What do you mean "incredibly unique?"
I mean that no other species has the ability to do the same.
> And what is the value in expressing the same thing in so many different ways? I don't see how you can argue that saying the same things over and over is intelligent.
Saying the same thing over and over again is not "intelligent." But being able to produce an infinite set of linguistic expressions which express the same idea is atomically indicative of a higher order of linguistic creativity than anything any other species is capable of. I'd say that qualifies as intelligence.
> They are really not very different from us in terms of linguistic abilities. Our linguistic abilities are mostly a matter of socialization of history.
I'm going to have to disagree in a major way with this. Humans have an innate, generative grammar that allows for a completely different system of communication and thought than that of animals. Animals, at best, may have something like words. But they do not have grammar. That's huge.
> Feral humans won't be much smarter than an ape.
Difficult to judge which would be "smarter." Secondly, not sure what argument you're making. You're saying that our linguistic abilities are not very different than apes, because they are socializations. But you also claim that humans without language are roughly as intelligent as apes. So, if language does not correspond to intelligence, does that imply that feral humans are just as intelligent as normal humans?
Feral humans lack language because the brief period of time in which the human brain greatly expands its linguistic learning capabilities (infancy and young toddler-hood) is spent in isolation -- critical linguistic facets of the brain are never exercised. That period of development does not come back. Their brains are simply underdeveloped, in a fundamental way. I'd say that qualifies as less intelligent.
I'd recommend picking up "The Language Instinct" by Steven Pinker.