The argument from marginal cases is really a family of arguments. You start with a spectrum of cases, perhaps thinking of every creature as standing in a line according to ability.
Cog and Chimp are just alike, as far as capacities go. From the fact that we feel equal concern for Norm and Cog, the argument tries to leverage equal concern for Norm, Cog, Chimp, and Mouse. Here's an example of this sort of argument (this is inspired by Tom Regan, but not directly from his writing).
(1) Norm and Cog have the same inherent value and thus the same basic rights, despite their huge differences.
(2) That must be because they share sentience (Regan actually talks about a slightly more advanced but ill-defined attribute--being a "subject of a life").
(3) But Chimp has sentience too.
(4) So Chimp has the same inherent value and basic rights as Norm and Cog.
(5) Mouse has sentience too.
(C) So Mouse has the same inherent value and basic rights as Norm, Cog, and Chimp.
If this were a good argument it would be prove a lot. If you thought Norm had a basic right to life, it would prove that Cog, Chimp, and Mouse do too. That would foreclose many things we normally consider permissible, like experimenting on Chimp or Mouse or eating them, if human survival were at stake.
Is it a good argument? We're to affirm (1), which is fine, but I think that affirmation is not simple. There's a lot going on in our heads when we think Cog has all the same basic rights as Norm. A lot of this is pretty recent stuff. If you go far enough back, disabled people actually were seen somewhat as animals are today. They were used in medical experiments for the benefit of the rest of us (not incessantly, but occasionally), and some were even put on display at fairs. Amazing, but true.
What we have now is a "championing" mentality. How dare anyone not give Cog the same rights as Norm? We identify with Cog, becuase we've been taught to stop thinking of people with disabilities as "other". We now realize that there are no immutable lines between "us" and "them." If I wind up in an accident or suffering from Alzheimer's disease, I will be disabled. 'That could be me" opens the door to all sorts of caring moral emotions. So--there's a lot going on in our heads when we accept (1).
But that stuff is thrown out at the next step. We are told that Cog's sharing Norm's basic rights rests on nothing but his being sentient like Norm. We are not allowed to see any complexity in our reaction--like its being partly a matter of seeing the capacities Cog does have, and partly a matter of sympathy, and partly a matter of wanting protection for our own relatives some day, and partly a matter of knowing that could be us some day. We are to abandon all of that and just settle for the idea that Norm and Cog have the same basic rights because they are both sentient.
But how plausible is that? Those who make arguments like this never explain how you get from simple sentience to immensely powerful rights. On the face of it, there's a huge gap to be bridged. Having basic rights is a hugely significant thing. Rights are like a magnetic charge that attracts and repels. If you have rights, that forces all sorts of adjustments on everyone else. At the very least, they have to "give you your space," however much that may inconvenience them or set back their serious interests. Sheer sentience can do all that?
In my book I imagine some tiny little creatures called Pangfish. They have limited sentience, consisting in nothing but the ability to suffer headaches. When the headaches go away, their consciousness fades out. That's their whole conscious life. I find it utterly fantastic to suppose that on such a slim basis they could have the sort of inviolability that would force me to choose death rather than eating them. Or that would force a medical researcher to let thousands of people die, rather than extracting some life-saving chemical from them.
So I buy (1), but not (2), and that stops the argument from successfully supporting the conclusion. I think we need a better explanation why Norm and Cog have the same rights, one that refers to things they have in common internally, but also to our emotions, "the social contract" we have as members of one community, self interest, and so on. I grant that's a hodgepodge, but I think it's better than the obviously inadequate explanation offered by (2).
What I'm saying does not leave Cog out in the cold (obviously). It also does not leave Chimp and Mouse out in the cold. The details are discussed at length in my book, but based on what capacities they do have, we do owe them respect, and that does force us to make careful decisions about what we may and may not do to them. Furthermore, sheer sentience does make some difference. We should not gratuitously cause pain. If I'm going to eat Mouse because my life depends on it, I shouldn't cause him more pain than necessary.
In short, I don't buy the argument from marginal cases in my book because I think it's very weak. In fact, I'd go further. I find it rather offensive, and so do the students I teach. The idea that some people are "marginal" offends them. People with disabilities have only recently been raised in status--in fact, in earlier versions of Animal Liberation, they are referred to as "mental defectives"! (Good heavens.) Most people in our society do think that animals are "just animals." So the "Cog= Chimp" equation is more likely to demote Cog than to elevate Chimp. Finally, it's inane to think that animals are like humans, but just less capable. Chimpanzees have capacities and a way of life all their own. My book champions animals without making any comparisons between them and people with disabilities, and I think that's all to the good.