One a week isn't a lot when you play multiple jobs. My PLD isn't 330 yet, but my WHM and SCH are both over it (but only 334, so not by that much). To suggest I don't know how to do the fight on one and do know on the other would be rather odd, since I got my first clear on the PLD. Yet, that's what these item level restrictions effectively do.
It is parser related though, since an argument anti parser people use is that having parsers would lead to discrimination on the basis of DPS. We already have that, we just have it on the basis of assumed DPS using ilvl as a proxy, which isn't exactly accurate.This, actually, is not parser-related and exist because there's "no official parser".
People like to play it safe, so they put a high ilvl requirement, so that they'l get mostly people, who "main" that class. Because if you "main" X class, you should be far higher in ilvl, than 330, even if you just collect Creation gear + Ivalice + those upgrades to 340. (aka doing zero Savages)
p.s. I also got 4-5 340 upgrades items from Hunts. Forgot about those.
I mean, it's a lot more useful to be able to say "we hit the enrage because you're doing 1000 DPS", rather than "we hit the enrage, must be because 325 gear isn't good enough for this fight."