Well if we had a lot more classes (like FFXI) I'd be OK with adding in something like Necromancer. As it stands there's just too many missing iconic classes and too many interesting ideas to add in something that's redundant.
I'm not just picking on you or even the idea of Necromancers. This sort of thing is also what bugs me about people who want Corsair, a ranged DPS/buff class. I mean, we already have one. It's called BARD. It's also why Dark Knight, as people want it, seems redundant. A heavy plate class that uses a 2H weapon to land slow but powerful hits... like a Warrior, BUT WITH THE POISON SPELL! Heck even Thief, a positioning-oriented light melee class is taken up by Monks (and Dragoons, in a way) thanks to the combo system requiring you to be conscious about positioning. The only advantage Thief would actually have over Necromancer is that Thieves have played a HUGE role in the Final Fantasy series compared to Necromancers who have had really small cameos.
This is why I keep bringing up Mathematician (and I keep calling him that instead of Calculator because that's the name used in the survey a while back) as a good possible addition -- a positioning, distance-conscious DPS caster with odd geometric shapes for AOE? That would be badass.