I think we should add variety, the frostweaver is a ranged dps. We dont know how they are going to work yet but im guessing its going to have to do with freezing people and slowing them at a distance (although i can see the comparison because death in crowfall=winter and winter=cold/frost.)
Frostweavers would probably be pretty good in groups, maybe even alone.
Necromancers need to be horrible alone, and great in groups.
Why do we need this archetype? As i mentioned above, variety. Thats mostly why we need any archetype. The frostweaver is going to be helping her teamates by freezing key enemies, putting ice spikes through their hearts, helping the offence, possibly defence by not letting anyone pass by without being frozen.
My ideal Necro will bill running with his group, taking adavantage of any enemy that falls and turning them against his own team. Overwhelming them into submisson with a zombie horde in the fray of the battle. Curing his teammates of in ill-debuffs and etc, and being like i mentioned before; a teamplayer.
Basically my answer is:
A cool support class that adds a different dynamic to the game. A complete teamplayer.
I feel that the addition of various titles given for completing certain tasks in worlds would work greatly with the game's concept of dying worlds. Such as getting a title that is available through a random world that way everyone can get awesome tags that go along their name. Ex: if your name is "Tyrion" and you stumble upon a title for completing some sort of milestone you would then get a title such as "The Destroyer" so you would be known as "Tyrion the Destroyer" (Not asking for super complicated titles or such. I just thought the idea would be cool to bring up.)