The necromancer only necros on the side. His real job is in a small accounting office, processing paperwork for Aetherial Inc.
Bummer, I was really hoping the necros would really be baristas wearing goth attire. My future plans as a necro are now dashed! 
Aetherial Inc. is an industry owned by Aetherial the user? If yes good luck translating those paperworks.
With higher levels / modifiers they last way longer then their cooldown so you can keep up the max. number of them all the time, all you need to do is press the button you bound the raise skeletons skill like every 10 or so seconds.
And as the great Greek philosopher Socrates said once: “If u don’t like hittin’ buttons, video games are not for u”.
But what designs would they make out of the top foam on your white chocolate caramel macchiatto???
Some kind of offhand hint about the necromancer’s real motivations? It would explain much if it turns out that Uroboruuk is actually something like Anasteria, except much more ancient and powerful.
Skulls obviously.
I really liked the necro in d2 AND Path of Exile. It’s fun running around with an army. Even titan quest had more than just 3 permanent pets. I would settle for multiple hellhounds (2-3) the higher level it goes.
The way things are you CAN run with an army. You just have to be careful about how and when you use your skills.
It really doesn’t matter if they’re permanent or not mechanics-wise, it really only matters that as long as you play your build right you’ll always have your minions around you.