One person helping is not really much help, lets be honest here.. 4 on the other hand is a huge difference. If you have do 2 full groups available, one group can run through, get their loot and then split into the groups up to give a far better chance to get the kill so the loot chance stays the same for both groups, 1 piece for 4 people... and there, penalty negated and carries/sales abound.

Allowing 1 person to help, yes, the loot chance for the remaining 7 does increase slightly, however that 1 persons ability isn't the reason they will take down an encounter. Also, for most groups, the person filling in is going to be of about equal skill. It's not as if someone from say Collision is going to hop in with a group that is struggling on the adds in FCOB turn 1, they are going to find someone that maybe has a couple turn 1 kills or even a turn 2's under their sash. Allowing 1 without penalty would not have significantly increased the rate at which the gear was obtained other then by allowing groups to more quickly find that one replacement for the night the rest of their group can all set time aside to play.

As it has been pointed out, loot tables in this game are down right near broken. Look at weapons, FCOB turn 4, each group has a 10% chance to get a weapon they need. A perfect world means it takes 2 months for each player to get their weapon and we all are perfectly aware that it takes far longer with repeats and non played job weapons appearing. My group had a player get a DRG weapon (3rd or 4th one we saw) well before they got their main job weapon.. and their DRG wasn't even 50 yet. We already are getting offspec loot from 10 so for the most part (starting to look like the first month of Coil turn 1 all over again), we are going in expecting only 1 chest to have something usable. So now, whats the point for us? One person can't make it a given week, why bother bringing someone in if the chance at something is approaching ever closer to 0%? Why not take our 7 we have and go do treasure maps and just mess around? or call it a early night and visit our families?

Also, as someone stated, this is it for ARR, no more after this, 4 man dungeon gear will replace everything we are getting in about 5 months from now. There are people who have yet to clear coil turn 5, let alone Scob 4 and how many are not going to get to see the cutscene after FCOB 4? Unless they remove the lockout in 2.5 (which I think would just promote selling runs doing that so quickly) whats really is the harm is allowing players to have that insignificant increase in drop chance and allowing people to play with friends that may be normally in different groups that happen to run different nights?