Sorry, I try to stay out of Keits' topics, but I finally reread this topic and just can't resist this one.
Group A - Group of 6 good members. Kills EMs at a rate of 1 per minute. This group kills 60 monsters per hour. They make 2100xp.
Group B - Group of 5 good members, with one poor member. Kills EMs at a rate of 1 per one and a half minutes. This group kills 40 monsters per hour. This group makes 1400xp.
Group C - Group of 5 good members. Kills ITs at a rate of 1 per one and a half minutes. (Under the reasoning that the 6th worthless member was adding very little, and wasting healer mana being cured from AE). This group makes 1440xp.
WTF is this mess? ^^ I think what you need to prove here is that you get more xp in a 5 man party than a 6 man party with a lacking 6-th man. You didn't do that in the examples provided. Let me do it for you. FYI, I'm also not including xp chains.
Group A - a good 6-man party: Irrelevant, we're not comparing anything to this.
Group B - 6 man group with 1 poor member. From FFXI Mystery Tour, a 6 man group will get 35 xp per kill from an EM. If it takes the group 1.5 minutes to kill an EM, then they will average 40 kills per hour and get 1400 xp per hour.
Group C - 5 man group doing IT. This was your flaw, you should be comparing to a 5-man group doing EM, not a 5 man group doing IT's. If this 5-man group can handle IT's, then so can group B. Afterall, group B is the same as group C, but has a DD who doesn't do a lot of damage. Again, from FFXI Mystery Tour, a 5 man group will get 37 xp per kill from an EM. You will have to assume the lacking player did SOME damage, so we'll say that damage can be made up in 1 attack round. Let's just say it's 6 seconds for an attack round (2-handed weapons have huge delays after all. I know my MNK attacked every 7-8 seconds, so 6 is a nice medium to get everyone to attack once.) So if group B can do this mob in 1.5 minutes, then this 5 man group can kill the same mob in 1.6 minutes, for an average of 37.5 kills per hour and 1387 xp per hour.
Now how is 1387 > 1400? Seems your math doesn't add up does it?
I'd also like to point out that eventhough a NIN/RNG would do more damage throwing shirukens, and eventhough you won't ever really see a NIN spamming shiruken, that a NIN/THF can spam shiruken and do extremely good damage. It would seem that NIN/THF being worthless is in FACT, an opinion.
I will have to agree that most of the time, a NIN/THF is worthless though. And that if you have a 5-man group, it is almost always better to wait for a 6-th man that can contribute to the party. If it's getting late (only a couple hours left to party), then by all means, go ahead and go with what you can get.
Forgot this one, the 6th man taking AE damage will NOT make that group kill slower than the 5-man group. I almost didn't want to discuss your AE damage taking up healer MP, but figured since you don't ever seem to wrap your head around the fact that AE damage is almost always cured with curaga, that I should point that out. If the AE damage isn't being cured by curaga, then your group is either taking too long to kill (multiple ae's in the same fight) or you don't have a RDM or BRD that can toss out the curaga and not pull hate. Of course, I'm spoiled from having a blink tank and BRD in my static, so curaga's are thrown out like candy at a parade in most of my groups.
Whichever way you lean on the "curaga" issue, is where the real debate here is, because if you don't have someone to throw out that curaga, then it very will could take longer for Group B to kill than Group C. But that's not something you can prove 100% of the time, and therefor becomes an opinion.