The issue with those was not that they're separate dungeons, but that they are either buggy as hell, and keep you dancing in circles forever, even when bosses hit 1 HP, or time waste that comes from people AFKing at entrance. Another issue was that vast majority of event dungeons had a daily lockout, despite dynamic quest giving nothing but a useless food item and about 10k xp (which could easily be moved to daily quest that usually gives event currency so that lvl 2-10 characters don't abuse it ((omegaLOL)).). Why would that matter? Because every time someone leaves, dungeon is either 15% slower, or more people leave, and it's impossible to recruit the missing player, since usually normal people don't want to be trapped in event dungeon that gets completed while they're in a loading screen. Or take the banshee dungeon for example. Some people care about actually completing it, while some might not, and just afk while most waves get missed. And instead of fixing these issues (most of them rather simple to avoid, really), NC buries their heads in the sand and give random amount of DC to complete while continuously putting more difficult dungeons into the rotation, and now forcing hard modes instead of giving free choice of dungeon mode.