I think Song can be good if your relying on some amount of opponent misplays. I also believe most people dont understand how this deck works, and thats the advantage of it. So far the 2 Sorcerous Spyglass are good out of the SB. Have a paper event this Sun. Looking forward to testing the new configuration!
Cards should never be run because we assume imperfect play from the opponent, but should be run on their own merit. Song of the Dryads is being run in order to diversify the sideboard answers. Enchantments are one of the harder spell types to answer in Vintage, especially non-blue ones that cost 3 mana. A Beast Within-style catch all was something that I want access too, and I chose Song over Beast Within because it has less of a liability. This deck, unsurprisingly, is better at dealing with a leftover non-basic land than a 3/3 creature.