Making the life taps insta cast would make sk invulnerable.
I disagree with you, however; I see where your going with your comment.
Some of the life taps can be cast without a recast delay but the good taps have them. The recast delay could be changed to better restrict the ability. I would put the half the taps on one cool down timer and the other half on another, that would allow an SK to use their spells at whatever level they are at and still only have access to two taps at one time.
and, or
To counter the instant cast, the mana cost could be raised to provide taps for strategic moments in battle.
