Not a bad idea, but if the mana-tap replaced festering it would make things worse. Tons of mobs dont have mana to tap, and it would go from being "we just wiped and FC is on cd so our necros are gimped" to "oh this mob has no mana necros are bad for this fight"
Ofc if you just made it feed the necro mana regardless of whether the target has any, this could be an option.
I also always liked the necros feeding the group mana thing, and this actually happens now in exp groups, but in raids its usually hard for the necro to get killshots, so they cant do that. A 15 min cooldown scaling paragon type ability could be very cool.
perhaps a low damage DOT that converts Health into mana at a 2-1 ratio.
IE dot does 300 damage a tick, for 10 ticks. Granting 600 mana to the caster per tick or something.