If an effect allows you to cast a spell "without paying its mana cost," you can't cast that spell for its cleave cost
However, the buyer of the card recently spoke to Kotaku about the sale, claiming that it was a completely legitimate transaction and that he feels $3 million is "about right" in today's market
You can land it early to kill Birds of Paradise and other cheap mana dorks while punishing your opponents for drawing cards, which Commander decks do in spades
Otherwise, players can get their hands on the expansion early with the Sword & Shield - Fusion Strike Build & Battle Box, set to be released in local participating stores on October 30th, featuring four booster packs and a 23-card Evolution pack, as well as four alternate-art promo cards and a deckbuilding tip sheet.
I dont think Dragonstorm was unbeatable, and the Mystical Teachings decks I'm referencing definitely beat it, but calling it OK to exist is probably stretching it