the idea is, if you want the weapon, you need to beat it as that class. seems only fair.
having something like this might also promote more diverse set ups. (if you want your mnk weapon, you're gonna need to play as mnk, therefore your most likely going to not turn someone away, just because they are a mnk)

also, I didn't suggest that you need to be all classes, all the time.
my idea was to either put it as an acheivement, so that people might want to give it a shot.
or offer a prize for your first or 20th even 50th time beating it with all classes. (I see 50 being a stretch and forcing people to only play as all jobs.)

I also suggested a contest, first group to beat primals with all classes gets a prize or recognition.

alright, if this will cause too many problems, how bout achievements for beating every fight as each job (say 5 times), another for beating it with an all class set up (5 times), and a reward upon completing all the achievements for each fight.

keep the random weapon chance, just incase people can only do burn set ups, random totems as well incase they can't play as the job they want, but of they can successfully complete a fight as any given job 20 times, they get their weapon.

for those 2 points:
1. there's no reason that MNK can't be successful in ifrit, fists of wind increases movement speed giving u an edge to the fight. ifrit is weak to wind, and it allows you to use your more powerful weaponskills more often (not really sure how much it cuts down the recast).
you'd just have to promise me you wouldn't go cutting off the mages :P
and I understand, ATM, there's just no reason to have a mnk when u can have 5xblm, but that's something I strongly believe has to change. This game is just boring to play with these stupid set ups.
every aoe fight is just fire fire fire, every single target boss is just thunder thunder thunder.
I think I've perfected BLM by now, I'd love to try another class >.> lol

for the other point, I'm sure they could lock you into the class u enter with and give u a warning upon going in. or lock your class once u attack, until u leave

**sorry for the lack of caps, doing this on my phone**