@ Ander Hammer
I admit my opinion may be a little slanted as to the communities view as a whole, however eh, still my opinion. (Also, did I not say that I would say something stupid, right, right! :P)
But anyway, the reason (in my opinion) N! exists is to create a system similar to the original Doom, where monsters respawned. The purpose of the challenge modes is to add specific forms of challenge, e.g. you're only allowed to use pistols. (AoMr)
If I decide one merry day to play a challenge called Angel of Darkness, I do not believe that I should be subject to N! standards if I wish to play on HNTR, HMP or UV. Although I admit that the respawning is just part of the challenge, there are other parts that must be considered. For example, it is not compulsory to clear levels in a standard game of AoD. It is however in Hell's Arena. The player must then employ N! style tactics to corpse-stack, etc etc. If he/she has never played N!, he/she is going to have more trouble than what Hell's Arena should be, if not being played on N!.
Now don't get me wrong at all, I'm not knocking those who can commit to those tactics, however it's my personal belief that AoD should not be as harshly subject to the conditions imposed on N! players (respawning) on levels where you are required to employ the tactics. If you're just on a level, trying to clear it, then the respawning challenge is something which is more of the "darkness" motif, not knowing where the next bad guy is going to pop out from. However in an arena setting, the challenge is not parallel to the challenge mode in question.
I also further admit that "employ advanced tactics" isn't the right contextual saying, I hope the actual meaning of my (properly used) words is evident in the above paragraph. (Perhaps, "if you are not playing N!, you shouldn't have to employ N! tactics." is a better way to put it)
I also note that this is not a personal/group/any form of attack against anyone, but someone asked me a question, and I am doing my earnest to explain my point of view. Apologies if anyone takes/took offence.