So It's against the rules to need for another job?....

[Niwashi]

It's still the fair thing to do since everyone there participated. That and it helps even out the RNG a bit. I mean, you could get only tank drops the whole run and the tank would just take everything. Where if he greeded, everyone would have a CHANCE at something.

Actually, no, it doesn't even out the RNG. It's as likely for the tank to win all the RNG rolls if everyone greeds as it is for only tank gear to drop. It just changes when the randomness comes into play, whether it's in determining which pieces drop, or in determining who gets them.

So long as everyone is following the same standard, overall gear distribution should be fairly balanced. Either each player has certain items they can Need on and a random chance of those dropping, or else everyone using Greed with a random chance of winning the roll against each other on each piece.

The times you end up with uneven distribution are when different players are playing by different rules. If some are skipping the Need option and just Greeding everything, while others are Needing what they can and Greeding the rest, then you'll get an unfair distribution. But that unfair distribution can't be blamed on one particular side. Both are equally at fault for not coordinating with each other what they're doing. (Unless, of course, they did coordinate a plan and one side broke it. That's the only case when there's someone to blame for unequal distribution.)

When playing with a random group, you have to accept that, despite whether it's your preferred distribution plan or not, the default plan the group as a whole has is the one established by the game itself. The game software has already established a rule that (baring a few special cases like Unique items you already have) you can Need on anything appropriate to your current class/job, and can Greed on anything else. So if you care about equal distribution, then that's the rule you should be following in all random groups. To follow any other rule, you should have a PF party with loot rules in the PF description.

If someone is rolling need for this reason I will leave

Now that I'm curious about, specifically the "for this reason" part. When someone else in a group rolls on something, how in the world do you decide what their reasons are? It's possible they might want to trade it in for seals, but it's also possible that they may want it for an alt class or cosmetic. Maybe they've been farming this instance over and over for weeks just trying to get this specific item because it's the final piece they need to complete their outfit. It might even be the only reason they're currently playing the job that can Need on it (which would make the suggestion that they shouldn't Need on it rather inane).

Now I actually agree with you when it comes to rolling Need to sell or turn in for seals. But I'd never just assume that's the case when somebody rolls on something. It sounds like that's likely what happened with the OP. He actually did need the gear as gear, not for seals or gil. But the rest of the group acted like he was being greedy to take his loot.