Umm... Why? Why do these things exist? I guess one could argue that it can make things like Aven Smokeweaver and Melesse Spirit unblockable/invulnerable to a creature, but it seems like a waste of a card. Horobi, Death's Wail can use the black version as an instant-speed boardwipe, but that's about it. I guess they can combo with cards like An-Havva Inn, Hellfire, and Yavimaya Kavu. Jovial Evil would be funny too. It's just... these cards don't do much in 99.99% of decks they're put in. At least Sway of Illusion draws you a card, but it costs double the other cards.
Pros: Can combo
Cons: Only has dumb combos, useless on their own