It's certainly conceivable. It's a plausible idea and a provocative one. It's an idea worth developing. It's about something substantive: what duties does someone owe to a person that's not fully competent? It's an issue with ethical implications for the disabled, for minors, and for that matter also animals. Examining it through the allegory of aliens is a potentially fruitful way to go. .
You have a very specific angle on this, that could be interesting, but why should they have to have that point to make, to include this kind of character?
But this? ``The whole trouble is caused by these inferiors not knowing and keeping their place''?.
Those bastards!! Wait, no, they never said that, or anything like that. Their attitude consisted of -- These guys aren't very bright or mechanical, so let's send Geordi to fix their ship. Oh no, now they're holding Geordi hostage." That's all.
When Riker said "You need to continue to develop" it was because the Pakleds had just put him on the spot, having just asked, "You think we are not smart?" It was more diplomatic than "You guys are idiots." And the Enterprise people's general point about the Pakleds was merely that it's not really practical to strike out into space when the engineering of your ships is beyond you, and you therefore have to beg for repair help from passers-by, every time the least little thing goes wrong.
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They're not mentally incompetent, not impaired, just not as smart as a lot of species-- it's what Pakleds are like at this point. Splitting hairs? I don't know. I just don't see this as an opportunity to make some point about how to treat the mentally handicapped. That could be a good script, but a very different situation. Here, the idea of a species of lower intelligence, which has managed to get into space anyway, is included as an interesting element or possibility, in the same way that, say, a barrier at the edge of the galaxy is. That's sufficient justification for them, though there are more interesting approaches.
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Our responsibilities toward them? Well, Riker doesn't kill them, which was nice. Low intelligence doesn't excuse kidnapping and violence to get what you want.