Pure support jobs are a NIGHTMARE balancewise, because you typically wind up with one of two situations. The first issue, which mostly crops up with debuffs, is that they're pretty much useless. Either too many things resist their status heavily (Or are outright immune) or their effects are small enough that just adding in another DD would be preferred.

The other example is what we saw happen in FFXI, where they're so powerful that nobody will want to do anything without them. Usually this is a case of buffs that increase damage (Since faster kills is generally preferred more than being invincible), but it can also be the case with downright broken debuffs. While it's possible to play without these debuffers, because they make all the party members so much better, it's considered not worth it to do an event without them.

Anyway, in FFXIV it seems what they've done instead is spread debuffs out across many jobs. You've got Virus from arcanist, slow from Lancer (via feint), as well as a wide variety of poisons you can use as items to inflict debuffs on a monster. Plus most DoW have their own self-buffs, and CNJ comes with party buffs of it's own. By spreading the support out, it's easier to balance without ruining a class. They can keep the buffs to an extent where they're useful, but not REQUIRED (Nobody would take a buffer class if the buffs were only "ok" rather than "essential"), while the class can do their main role in addition to their buffs.