Typically the kinds of deck that want coverall counterspells also want to leave mana up to be reactive, having to tap five mana in your upkeep will make the turns following a Pact of Negation deeply uncomfortable until you have lots and lots of mana
Exploit means When this creature enters, you may sacrifice a creature. 702.110b: A creature with exploit exploits a creature when the controller of the exploit ability sacrifices a creature as that ability resolves.
Some planes will let you draw cards, while others can destroy everything in play when you encounter it
Another favorite card of mine, Prophet of Kruphix, can really provide a platform for this to shine
So let's see which cards with the ability get played the most in commander
This triggers creaturefall because the card left the battlefield and returned as Ifrit, Warden of Inferno, which is also a creature