Nobody uses it because Ent is a much better purchase.
I think your idea could be useful, but not as a Common card. It's highly situational, and in the event that it is used, it is unlikely that the necessary condition would be met by the opponent a second time. Anyone including it in his or her deck would be discarding it almost every time it appears.
My suggestion would be to either make it less than common or to add another effect to it, such as adding one to wall or to a random resource or reducing the opponent's wall to a very small degree, such that it isn't completely useless when the condition is not met for the full effect.
I also like to consider how proposed free cards affect Danai present. Since this is a situational card, I feel it would have the overall effect of making Danai present more useful to the one playing it, so it passes that test. =)