Toils of Night and Day

The Arcane subtype here points backward, not forward: this is a host, an instant built so that other spells with splice onto Arcane can attach their text to it, rather than a rider grafting onto someone else's cast. Read as a standalone spell, the effect is thin. Tapping or untapping two permanents at instant speed lets you stand two lands back up to leave an instant uncast, freeze two attackers before they connect, or set up surprise blockers, but none of those plays bends a game on its own, and at three mana the untap-lands line is closer to mana-neutral than to a windfall. The cost is really being paid for the type line: cast this, and any number of cards with splice onto Arcane in hand can splice onto it, turning a marginal utility effect into the seed of a much larger turn. The optionality lives in the action, not the selection. You commit to two permanents when you cast, but each "tap or untap" is a may, so the spell resolves cleanly even when one of those targets has nothing worth doing to it. That flexibility matters precisely because the card's job is to be a legal Arcane shell for everything you want to bolt on. Take splice away and almost nothing remains: an instant whose worth lived entirely in being the thing other spells could borrow a casting from.