Bob, 8, has written an essay for school. The teacher reads it out aloud to the class, and then tells everyone it's a stupidly written essay, full of spelling mistakes and the storyline is silly too. Does this make Bob learn to write better essays? Does it help if the teacher also goes through every mistake Bob made and suggests improvements?
For most instances of Bob, it doesn't help him to be taught that way. Some instances of Bob don't care, and will learn anyway, but those instances are in the minority. The majority of Bobs will be upset, and hurt, and will have a hard time accepting any lessons from the experience, and indeed may well have a harder time accepting any lessons from that teacher in the future.
Now, it's true that the emotional lives of 8-year-olds and adults are different. On the whole, adults can take more emotional stress and conflict, but the general effect is the same, just less strong.
There is a subset of programmers who have what is called a thick skin: they can take a lot more abuse than the average person without being upset much. Given the way text-only communication over the Internet tends to filter away subtle emotional expressions, it's very easy to express yourself in a way that seems neutral to you, but feels hurtful to the recipient. Without an effort to avoid that, mailing lists for free software development tend to push out those who are more easily hurt, making the list be mainly populated by those with thick skins.
That's not a good thing. Apart from any moral aspects, it reduces the number of potential participants a lot: those who submit patches and feel hurt, and those who are watching and decide to not even submit a patch.
Note that this is all about how you express criticism, not at all about having to be accepting and supporting of anyone who submits any kind of patch. It's perfectly fine to criticize and reject patches. You can, however, be nice about it.