Ah-ha, I found the place I got the list from:
Not Again: 24 Great Films Too Painful To Watch Twice | Film | Inventory | The A.V. Club (warning: page has many youtube embeds)
It doesn't seem the list is made in any particular order, otherwise I'd agree that some are out of place.
At least for the movies I've seen, I agree that they can actually benefit from multiple viewings, but I think the issue this list is getting at is that most people wouldn't want to go through the negative emotions the movies elicit more often than necessary. In the case of
Requiem for a Dream, and especially
Irreversible, I required significant breaks between viewings. I'm someone who generally doesn't mind and even enjoys depressing movies, so I can definitely imagine how many other people would simply rather not see the movies ever again. And yet, I think they're totally worth seeing at least once and are two of my favorites movies of all time.