The reason he's saying this Leeloo is because the job system won't *replace the class system, if it's just added to the game. Many ppl may choose to continue playing on their classes in favor of more "versatility," when what it really amounts to is little more than an undefined job. In these kinds of instances, it will be even more difficult than normal to guarantee what kind of player you're adding to your pt. Imagine if you do a search for a gladiator, and you ask him/her to come tank. They say yes, but when they get there, you realize that they only have Provoke equipped. They don't have Taunt equipped; they don't have Defender equipped; they don't have any Curing actions equipped; they don't have Second Wind equipped; or anything else that even remotely resembles a "tanking" class/job. All they have is a bunch of Melee actions equipped from LNC and MRD or something. You kindly ask them to put on some other actions, and they say: "I don't have those, and I keep hate good enough with DD'n."
Then, you're forced to argue with them about how it's much more of a pain to have to constantly spam them in cures because they're taking hits like a naked DoM, and you don't want to have to play *only Healer on your "healer" class; and so on. It's an endless cycle of ppl wanting to play their class an extremely "unconventional" way that doesn't really work out anyway because it's too hard for SE to maintain balance with so many combinations available to each class.
We all would absolutely LOVE the idea of infinite masteries and cross-class options. However, it's just not a practical. So, what is the solution? Do we put our trust in SE, and hope XIV doesn't fail us again? Do we put our trust in Yoshi-P's unparalleled genius, and hope that he can bear this burden all on his own w/o our input? Or do we make suggestions based on evidence from previous experience with other MMOs? Personally, I'd have to go with the latter.