Vampiric Touch: Could it be the Answer to Annariel's Mana Issues?

Annariel hit level 63 last night & with it, she finally obtained her 41 point shadow talent, Vampiric Touch. Now, the tool tip for rank 2 of the spells says it does 600 shadow damage over 15 seconds & the party will gain mana back equal to 5% of the shadow spell damage caused by the priest. This effect appears to apply even if you are solo...so you don't have to be in a group to get the benefit. I now use this spell to pull with instead of mind blast so I can get the full effect of the mana regeneration. However, is 5% mana regeneration any good?

Here is a simple example. Most of the mobs Annariel is facing now have about 5 to 6K in hit points. So lets use 6000 hit points for this example. All of her damage, except for her wand, is shadow damage. So assuming she doesn't use a wand and is able to keep Vampiric Touch up for the duration of the fight, she will do 6000 shadow spell damage. That means she will gain back 300 mana during the fight. Since rank 2 of vampiric touch costs 400 mana, it doesn't seem like much of a bargain. Right now, I'm not too impressed. However, in using it yesterday, it did seem like she was able to go a little bit longer before having to drink, so maybe I am missing something. I guess I'll just have to keep testing it to see if it allows Annariel to become more mana efficient.