I tried a Necromancer/Dark Templar over the weekend with good success.
I got several good +minion items, so I was able to switch over to mostly minion skill by level 8. Possibly not optimal, but I really wanted to get into using summons.
I had actually done an Ice Mage/Necromacer in Din's, and found it fairly effective. I replaced Defender with Necromancer as a defense tree, using summons to take the heat and provide a bit of extra DPS.
It was effective, but I ended up going back to defender, because I found myself getting oneshot too easily. With Pain Delay in Zombasite, that is probably less of an issue.
My Necromancer/Dark Templar is level 11 now. I have 2 melee skeletons, 1 bowman and 1 zombie. I mostly save my mana to create full health skeletons as needed, and contribute DPS with my two-handed +minion damage sword, while enemies focus on the summons. I can't take handle damage myself, so if they start to focus on me, I end up kiting and using bolt.
I can see this build getting fairly powerful over time.
One of the issues with Necromancer is that summons don't persist on save/load. I raised a really nice yellow zombie yesterday, just before I had to stop... too bad. Would be nice to have a fix for this, and buffs in general, possible in an expansion pack?
I'm not sure what buffs I will end up using for Dark Templar. It seems this tree is more powerful late game when you already have a good base to apply modifiers to, so I don't see myself investing there for a while.
The +% vitality is obviously good. I'll have to see if +%armor and defense end up being worthwhile for a summoner.
I'll mostly likely skip or do a minimum investment in +% damage. I have found myself using a sword still early on, so a bit of extra damage might be worth it.
For stats, I'll want to invest enough to get superman and pain delay at least.
The traits that allows you to share health/defense with minions could be very interesting as well. I'm not sure how Pain Delay will interact with those. If sharing health, I might end up taking too much damage when hit by an AOE, although it would potentially allow a self-heal by replacing damaged summons with full health ones.