Are some raid bosses just too hard?

Mank alerted me to an interesting story in The Escapist, where one of the top raid guilds in Final Fantasy XI gave up on a raid boss after 18 hours of nonstop fighting, as people started to get physically ill from playing for so long. Apparently there are very, very few raid guilds who have ever killed that boss without using some exploit. Which makes me wonder why such a boss is in the game at all. What purpose does content serve which can't be beat?