I totally agree that the "melee + white magic + black magic" combo as their main focus really isn't viable. I play hybrid classes in most games when I can, and I don't think people appreciate quite how difficult those are to balance. I'd be totally happy with an enfeebling "magic fencer" style class for RDM in FFXIV whose primary skills involve debuffing weaponskills and/or spells.
That said, the thing about enfeebles is that unless their duration is super-short, you're not going to spend *all* your time doing that. Buffing is an important part of being a FFXIV BRD, but they spend a lot (most?) of their time dealing damage, unlike FFXI bards. Enfeebling can be a core part of FFXIV RDM while still leaving room for DD or some backup healing. If you gave them access to CNJ and THM as subs there are some tricks you could do around the edges to make it "feel" more like a traditional RDM without trying to make it a full on hybrid. With only one cure spell and the first tier nukes, they're never going to be primary healers or nukers. But, honestly, it could be a simple as giving them a weaponskill or ability on a 30 or 45 second cooldown that "empowers" the next first tier heal/nuke spell to hit more like a -ra spell... that would go a long way towards making the class "feel" more like that hybrid mage without really making it one.
I don't think we really need to worry about them making a debuff class and having it be totally useless for those reasons. I also suspect that they won't want a debuff class to totally step on the toes of the debuffs that other classes have on their WS, and I suspect when those debuffs were selected, they were done so in consideration of what a future debuff class might be able to do.
I could see a situation where more mild debuffs (stat down, slow, etc) could be applied throughout the battle, whereas certain strong debuffs (silence, paralyze, sleeps, etc) were subject to resists and so you'd need to make interesting decisions about when to use them on harder/longer fights.