you have to give it a special trick in order to shoehorn it in, the after-effect likely being having to go back and re-adjust other jobs to compensate, again, and again, and again.
I don't really think that giving everyone something to "shoehorn" them in is so bad. It's kind of what wow does, the problem with wow is that every expansion they add new abilities to pretty much every single class, make adjustments to how stats scale, sometimes change how bosses stats scale, etc all while trying to balance between both pve and pvp. At that point there's so many variables it ends up taking a while through the expansion to get balance right.
FFXI doesn't have a big element of that, and that's pvp balance. Giving everyone their so called "shoehorn" ability/spell, is almost exactly what the game has to have for every job to be valuable. You don't want to create a situation where job stacking is the best way; bring 3 of this job as a tank, bring 9 of this, this, or this job for DD with one exception, a thief, maybe. The rest is support.
Every job needs something that buffs the rest of the alliance/party to some degree. And those abilities/spells all need to not stack with eachother in the sense that you don't want it setup such that if you bring 3 DRKs, your whole alliance's attack is increased by 10% * 3 DRK. In this way, if you bring one of everything, your alliance is the strongest that it can be. And if you bring 9 best geared DRKs, your alliance is sitting pretty low on the totem pole, no matter how flavor of the month DRK is.