We do not know if we can or cannot have permanent pets. Spirit Mark looks to be a toggle spell. We have not seen its upgrade yet...
Also, there is nothing wrong with the Necromancy tree now..We apparently learn it from Mortalitasi who are Nevarran mages which means the magic they know might not be similar to Ferelden Circle's or Kirkwall's or Witch of the Wilds' magic classification...
Here is Dorian's Necromancy skill tree...
Spirit Mark can be toggled. I hope the upgrade gives us permanent creature that we can toggle on or off...Hopefully...
Well its not the same if you read the descriptions on how they do the spell, horror is a good example. Since I played a necro mage in DAO, you don't use a spirit aka demon of fear to cast horror like necromancer in DAO. Plus you are binding spirits to create a mimic an exact copy of the person who is marked and doing damage at the same time. Which I think is more effective since now you no longer have to kill the person since now you still get you puppet and you kill them at the same time. Plus the spell animate dead I personally think you are not shoving a spirit into a corpse instead you are manipulating it like a puppet using mana.
Considering in normal circle towers the only way to get enough power to do that is through blood magic since they have no understanding how to do it properly like the Tevinter and Mortalitasi going by the lore and reactions of anders in DA2 with the undead.
Here is how I see it.
The base spells are similar to what we see in DAO but what we see of them within the specializations is but the is a higher understanding of said spells or advancing or changing them in some fashion or how to make them more effective which coincides with Mortalitasi lore. Since they are the only ones pushing the limits of magic apart from Tevinter.
In DAO horror is just a manipulation/forcing of the mind to see an illusion while necromancer is manipulating a spirit to do it for them. Plus I don't get the idea where people are thinking necromancer is only about skeletons and zombies when they are known to manipulate souls and spirits as well in other fantasy games. Oh and this necromancy tree coincides with how necromancy works in Thedas since you need to learn how to bind and summon spirits properly to control the dead.
Since how you learn the specialization in DAI or a part of it
Oh and small note as someone who normally plays necromancer in rpg games I am glad bioware went the spirit/soul manipulation route since people tend to forget necromancers can also do that in most worlds plus I find Thedas necromancy the most interesting.