Duplicate it a few times over via any number of duplication mechanisms (common enough in red or blue), and playing one spell would get you multiple creatures back. Since those creatures themselves are spells, it'd be too easy to set up a loop or combo. Making it legendary means you can't easily end up with "play one spell, get back N creatures".

For that matter, creatures are cards, and there are effects powered by discarding cards or cards going to the graveyard, so if you can play one spell to get 5 more cards in your hand, and then discard 5 cards to get an effect...