Lol, that will teach me to post late at night without carefully reading the post. I was thinking about abilities and didn't quite click to the context of a discard spell...
All card plays from hand require moving the card to a new location to register the game action, and having a particular set of spells that didn't would be inconsistent and jarring. (Even if you think it wouldn't be, I promise it would.) Not to mention, a one-input spell would lead to a LOT of accidental casting. We'd see complaints about the game "casting spells for me" every dang day.
Now, one does wonder if spells that target an opponent could be made to accept the entire playmat as a valid target, so you don't have to be as precise. That's one that could be worth it...