The thing is Necromancer in this game seems to be very support oriented, hence why you have not seen many who use it as a specialization or create builds for it.
This is one of the cons of creating a game centered around action play. The game then becomes all about killing the enemies as fast as possible. Damage over time does not help in killing enemies as fast as possible, burst damage does.
Necromancy in this game is about damage over time, small crowd control, pets and time alteration. They can have impressive damage numbers, but only over time. They have zero and I repeat that, zero, burst damage capability.
As such, their damage will be very consistent and good but it will be a plateau as opposed to bursts of damage.
Contrast this with Rift Mage which has crowd control and debuff in all its basic spells and their nuke, Stonefist, can be spammed.
Necromancer is better than Rift Mage in a few aspects however.
One is that Spirit Mark and Walking Bomb work on pretty much anything. Even the Horror damage over time, which you get by upgrading it, works on anything, it is only the panic part that does not do so. Contrast this with Rift Mage where Veilstrike and Pull of the Abyss absolutely do not work on large enemies such as Giants or High Dragons.
The second is that the Necromancer's Focus ability is far more reliable and more useful than Rift Mage's Focus ability. Firestorm sounds good on paper but in practice, it procs in a very irregular fashion, it does not do jack on fire immune or fire resistant enemies, enemies can get out of its blast radius and it can result in a total party wipeout if you have friendly fire on. Haste on the other hand works on anything and everything, procs all the time, is not affected by immunities or resistances of any form, friendly fire is not a concern and it is a spell that benefits the whole party.
The third thing is that Necromancers do not have to worry about dying. With Simulacrum, they laugh in the face of death whereas Knight Enchanters and Rift Mages cannot do such a thing. Simulacrum grants you lichdom which allows you to close Rifts, revive other party members and cast spells without using up any mana at all for 10 seconds. That basically amounts to 10 seconds of godhood. So if you are a Necromancer with Simulacrum passive and you are in a tough battle which results in a party wipeout, it will not be a problem for you because, well, you cannot die that easily.
The fourth thing is that in my opinion, Necromancer has really good animation and sound effects for their spells. Horror, despite its not-so-outstanding nature, looks awesome and sounds awesome to cast. So is Spirit Mark, especially if you manage to get a giant or a large bear.
The fifth and last thing is, well, nostalgia and sentiment. Only with the Necromancer specialization can you recreate some form of DA:O and DA2 mage in the game. What do I mean by this ? Here is a skillset for a Necromancer
- Horror
- Spirit Mark
- Walking Bomb
- Immolate
- Winter's Grasp
- Chain Lightning
- Barrier
- Haste
In DAO or DA2, these skills would be
- Entropy School Spell
- Spirit School Spell
- Spirit School Spell
- Primal / Elemental School Spell
- Primal / Elemental School Spell
- Primal / Elemental School Spell
- Arcane School Spell
- Creation School Spell
You cannot recreate a DAO or a DA2 mage using the Knight Enchanter or the Rift Mage. Neither of them have Entropy spells.
All in all, in order to play as a Necromancer effectively, one should be willing to
- Play a semi-support role. With that skillset setup I had above, you can be the only mage in the party.
- Not be afraid of dying.
- Not care about high burst damage.
- Be content with the potential disapproval you will receive from followers and advisers.
That means it requires a completely different playstyle to Rift Mage and Knight Enchanter. This is something that the majority of the playerbase has not gotten used to yet.