A lot of cheap white removal cards are temporary, only taking cards out of play until the removal itself is removed, like Seam Rip, which can also only target nonland permanents that cost two mana or less
Halftones are a necessity when printing on paper but not for viewing on a PC
If you counter one of your own spells then you get both the draw triggers giving you all three cards in the next upkeep
Since Polymorph decks are built around cheating creatures onto the field as opposed to casting them, the restriction of not casting permanents is hardly a downside
Once you've announced that you're casting a spell, players can't take any actions until you've finished doing so
These effects are powerful because they wont just get a creature from your graveyard directly onto the battlefield, they can often do so at a lower price than that creatures original casting cost