IIRC Bahamut's Tear in VP1 is the only case I've heard of where many people make an exception for "not enough to pass around".
Bahamut's Tear is a bit of a weird case, in fairness, partially cause its a Storebought weapon, used by only 3 PCs, in a rather sizable cast, but there's only two of them...
But in game, when Bahamut's Tear finally appears, there's...a boat load of other good, probably better, weapons to pass around. Cromrea, Gram, Valkyrie Favor (...ok, not to much this one, since while stronger, less attack trust and no +30% damage? Granted, wouldn't shock me if it was a superior weapon on Grey, given his horrible PWS and his above average attack mults, so...well, someone wanna math this one out <.<?), eventually Levantine, etc. In game, using 3 Greatswordsman, as redundant as using Kashell *AND* Arngrim in the same team is (Grey at least has Evade ignoring!) doesn't actually hurt the others as much as it sounds since usually there's enough of the good stuff to go around.
However, in the FF3DS case? Using Knight end game, assuming no Wifi related stuff (JL99 equips and Ultima Weapon mainly) pretty much requires him to use Ragnarok and Excalibur. This means you can't use Dark KNight (wants Ragnarok), Freelancer (wants Excalibur...admittedly there's 0 reason to use Freelancer, but that's an aside), Warrior (Ragnarok again), or Red Mage (Excalibur again)...well, in Red Mage's case, you could use him as a pure defensive healer with dual shields, but that still factors in that using a Knight limits Red Mage's usability by forcing him into a specific mold, when one of his big advantages in game is his variety and ability to take on most roles, while not excelling in any of them.
I have more respect for allowing, say, Dark Knight to use Masamune than giving Knight either Excalibur or Ragnarok. You could argue that Ninja is fast enough as is, and often using Throw and such ANYWAY, so Dark Knight can get away with Masamune without hurting Ninja too much...and even THAT'S a pretty big stretch (one that there's...no way I'm willing to follow it <_<)
Like Elfboy said, it works both ways, really.
Its an advantage of Dragoon that he's the only one who can use Lances. Lets him use whatever he damn pleases in game, weapon wise, and no other class has to suffer; only time you might care is if you really want to use 2 Dragoons. The fact that he can use Gungnir, let alone get it early, and no one else can is a huge boon for his worth.
Its an advantage of Vikings that only they can use Hammers; those two Blessed Hammers with that 120 Power relatively early were pretty rocking, and with no one to fight over them, it makes Viking look that much more feasible (didn't hurt that Axes were pretty much all store bought outside of the Dual Axe, so Viking rarely had to fight over them with Warrior, as they both could use them if need be.)
Knight, though? Pretty much the entire game, he's fighting over Swords with Warrior, and later, Dark Knight. I think Red Mage can use the lionshare of them as well, though unsure (when I use Red Mage, he equips a shield, and sits in the back row item casting Rods, so I tend to forget what swords he can use before Defenders.)
Black Belt has to compete with the much higher JL Monk for Claws too. Oh, yes, Black Belt has higher stats, but JLs matter so much, those probably won't make up for the stat difference here (now, in the case of Ninja vs. Dark Knight, the speed difference is so dramatic, there is an actual difference. Furthermore, the JL difference from Monk -> Black Belt is far bigger drop than Dark Knight -> Ninja, since one is a class gotten right in the beginning of the game vs. class gotten roughly half way in.)
So yeah, while the mind set is "I'd never use a Warrior and a Knight together!" or something...doesn't this logic apply to many cases? The very fact that you WOULDN'T is based around how the two hurt each other when used together; the very fact that the two of them CAN'T be used together without one being hit is a flaw to that class, where as classes like Viking and Dragoon allow full flexibility should you wish to use one of them.
