Checking the detail of the skills again, Michael realised he might have overlooked the effect of this skill his undead primarily used for combat.
For one they'd make excellent farm undead with the growth acceleration effect of the skill.
"Life force drain is also interesting. Though they might end up figuring how to use it on the future, as their master I should have paid attention to this things."
It was like how one trained their pet. The pets were naturally capable of doing what they're being trained to do but most likely wouldn't for a long time without guidance.
In the case of his undead, they had always been used for combat right from the start they became undead.