Poison is more difficult without curses, but there would be no reason to play them that way. Lower resist requires a very small point investment (1 + prereqs).
With lower resist? They are, easily, the most viable necromancer build for PvM.
That's a somewhat above average expected base damage for poison nova on a geared necromancer (one specced for pure damage can expect 11k; one specced more for utility can expect around 8k. One with bad gear or untwinked will be closer to 6k). But it's not just that: Lower resist gives the necromancer an integrated way to break immunes and pretty much double the damage he deals to any non-immune (if a monster has 50% resistance, it now has -10%, meaning it more than doubles it; at 75% it practically triples the damage). As well, with facets and death's web, you can expect an additional 60% from your gear. Death's web is expensive as hell, but even if you take it out of the equation, there are cheaper items (like trang's gloves) which give an obscene boon to poison damage.
But there are a few poison immunes that you won't break, and for them you have the remainder of your skill points to allocate for. And, moreso than basically any elemental damage-oriented build in the game, you really do have a lot to work with. As mentioned previously, they can essentially work a full skeleton build in. Or they can pump CE and focus on killing non-immunes to kill immunes.
Poison necros are one of the most viable soloing classes simply because they are capable of breaking immunes better than any class except for the Paladins and they can do so without any gear at all. They have no trouble dealing damage (mine is competitive in eight-player games, and in one player games everything dies in one nova except for act bosses, even broken immunes) and they come with a few tanks, too.
Also, skeletons, on the same necromancer (as you can see, this character is NOT specced for skeletons)...
Believe me, they'll be fine.
