Totally agree with you there and it was mentionned lots of times on the PTS but again BW didn't listen.
But something really "funny" happened the last time we tried it.
Basically we were 1 group of reps and 1 of imps trying to kill it, taking turns after each wipes.
After a while we noticed a sorcerer running around naked while the imp group was fighting the boss which was a bit weird at first.
But after watching more closely it turned out that he wasn't in the group that was fighting the boss but was reviving fallen members of the group by having the alphawave inhibitor buff.
Considering he was out of combat he could revive the tanks that were dying because of gore debuff or the dps that kept attacking the Thing after they got the blood mark.
Personally I'm not gonna give the guild name or players name, I don't care how people choose to play the game but I don't think that's fair for other people/guilds that don't have 16 man upfront for it and take time to gather people around and also for people that are trying it "the proper way".
We can all see now that: No BW, not everything works as intended...
I don't think BW intented for that kind of exploit too.
So again BW should listen more to the comments on PTS, I don't think it should take much to put the Nightmare Pilgrim in an instance where only 16 people would be allowed to enter and would solve also more problems like the huge lags when you have a lot of groups around.
It would be like EC you have 1 group per instance and it gives everyone the chance to kill it at any given time and not having to wait for turns/respawn.
I guess that revealing this exploit might give other people ideas but if it makes BW fix a couple of things on this boss well might as well do it.