I'd say that kind of difficulty should exist, but beyond the progression of gear, and should award some special titles or achievements or mounts to show off. This way mediocre players would not feel gimped by the difficulty of these bosses and hardcore players can showoff their titles.

Sort of like WoW (Yeah, yeah, yeah it's WoW again. But admit it, WoW is currently the most babysitting MMO for noobs and casuals, but STILL give hardcores something to enjoy.) WoW had something like that. It's sort of meaningless to list examples though, coz if you played it you know, if you haven't played it you won't know. Anyway for those that haven't played it, and are interested to know, there are fights that are just too random, you always don't get the chance, and killing the boss all depends on how well your team grasps the opportunity that happens once in ten fights. A good example will be "Sinestra", you can just search it on YouTube.