This means there are, supposedly, 15 total copies of these cards in circulation, or 5 each of the first, second, and third-place variants
It plays best with decks that can sacrifice the Druid to reliably use the death trigger
Well it seems they're catching up with Prismatic Evolutions print runs, as TCGplayer now has booster packs for $7.75 each
One of the reasons this card was banned was that the synergies with Manifold Mouse were too strong: They allow you to target your Heartfire Hero for free to trigger valiant and give it a +1/+1 counter, while you also give the Hero double strike for a turn
Also, sliver decks tend to go pretty wide so being able to tap all your creatures to draw a card, while also dealing massive amounts of damage each time is busted
That tracks with the 28% estimate I came up with three years ago when MH2 came out