This Aura is an extremely popular Commander card, listed in over 9300 decks according to EDHREC. Turning any permanent into a basic Forest is pretty mean for only 3 mana. It's essentially removal for any permanent on the table that's giving you a hard time.
Is it really a $10+ card, though? It's been $5 for a little while. Then there was the massive buyout with one copy left listed for $10,000, because someone was apparently being funny. It stayed around $12 for a couple of days as the dust settled and no one really bought any copies above $6 on TCGPlayer, according to the Market Price.
With new copies incoming from the Commander Anthology in June 2017, Song of the Dryads would settle in price return to around $5 or less again before slowly growing again for a long time. There's definitely demand for this card, as pretty much any Green deck might want this. It's actually better than Beast Within, in my opinion, and why not play both?