Not having read other responses to your posited scenario:
In the old-folks scenario, we have a classic movie presentation of an Eskimo (or Inuit) family wherein the grandparent has gotten to the point he/she can no longer chew on the hide to make it wearable and so is no longer a productive member of the family. According to the movie, at that point the grandparent went out alone on the ice to be eaten by a polar bear so that that bear could then be killed by the family and eaten, thus continuing to provide for the family.
OK, that was a single family (and Hollywood). A single family or a very small tribe would be living on the edge, such that any single non-contributing member could seriously jeopardize the survival fo the entire social unit. Any social unit that is living right on the bloody edge of sheer survival cannot tolerate any non-contributing member. But that's not where every tribe is, now is it? Desperate measures are needed by social units in desperate situations, but they aren't all there, now are they?
Now, how can the elderly contribute? One obvious way is in helping to raise the young. Grandparents? Ever hear of them? Wouldn't keeping the grandparents around to help care for and raise the children be an advantage? Wouldn't you think?
Now, think of the very word, "elderly". Who is usually presented as being the sage counsel who run the entire community? The "ELDERS". One thing about the old people is that ... THEY KNOW THINGS! Our present society has lost this, because so much of the pertinent information in our present societies is only weeks or maybe months old (FaceBook? What the frak is THAT? -- and now even FaceBook is old), so only the young people know anything about it. But traditionally, the old people had all the information that the society needed. Who sits in counsel for the entire community? THE ELDERS! To indirectly quote from a popular US sit-com, old people KNOW STUFF! -- Cloris Leachman in particular. In the traditional communities where things didn't change that much, you learned everything you needed to learn from your (same-sex) parent and your parent had learned from their (same-sex) parent. So it was extremely beneficial for your entire community to have your and others' grandparents available. Because the only source of information you had was what was passed on to you ORALLY.
OK, so we have two different tribes. One tribe got rid of its elders when they could no longer contribute materially to the tribe, whereas the other tribe kept and maintained its elders. A situation arises which had happened before. The first tribe doesn't have a clue what's happening, but the second tribe's elders had lived through the same kind of situation before. Who has the advantange?
Now, in your revised version:
quote:
Imagine the same two tribes scenario, but instead we are talking about deformed and/or weak babies.
Now, the situation in such scenarios is necessarily how close to extinction the tribes are living. If they are right on the very edge of dying out, then keeping such babies would be the kiss of death. But if they can survive in spite of keeping such babies, then the extra benefit of such sensibilities would benefit them.
Now, if that has escaped you, in dire straits, then dire measures are needed. But if you have lee-way, then taking that leeway can make your society a much more beneficial society for everybody involved.
So, you can dream up all the dire straits scenarios you want to, but for social situations where those dire straits do not apply (which is most of them) then your scenarios would not apply.