The Spirit Mark power has issues with minion AI to make it not worth the ability point; even the DoT capabilities of the power pale in comparison to the Walking Bomb power.The minions that you create with the power have horrible pathing and targeting priorities, and by the time your pet does decide to attack something it's timer is almost out or the party has already cleared the room of enemies.
Really, the only 'good' thing about Spirit Mark is it's trolling capability (if you consider trolling to be a positive asset); making a pet of the last guy in a room, or of the guardian of a chest will prevent the key from dropping, or the chest from opening until the power ends…. and the upgrade for Spirit Mark makes the pet last up to 45 seconds.
It sucks. I was in the same boat OP, I scrounge up all of my cloth to unlock the Necromancer, and charge directly down her Gravelord tree so that I can get my undead minions, and I discovered how underwhelming/borderline broken the Spirit Mark power is. The Necromancer is great for tanking, and spreading a never ending plague of Walking Bombs (with the right upgrades), but don't look to her for your "minion master" fix, it will only disappoint you. 