Like most Magic: The Gathering cards, the value of Frenetic Sliver depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Secret Lair Drop)
While it costs a hefty six mana to get online, once it is in play you get to add twice as much mana when you tap any permanent for mana
I would actually argue that Armada Wurm is the more proactive of the two and that Centaur Healer is more of a sideboard card (though it depends substantially on the tenor of the metagame)
Basically, it can attack right away, doesnt tap when it does, can only be blocked by two or more creatures, and if it does get blocked, excess damage hits the enemy player
Cards like Raffine, Scheming Seer, and Ledger Shredder show how powerful the mechanic becomes when built around properly, making connive one of the most skill-driven and engaging abilities Magic has introduced in recent years
With all that power, youll want to be able to cast whatever spell you want, and Vampiric Tutor does just that
