Not much of a plot device if it doesn't affect the plot. It's not like Doug needed an invitation to crash in any of the other worlds, after all.
Personally, I'm expecting Ginny and Hermione to mention it to Doug, and when he reads it, he'll say, "Well no wonder you didn't summon anything: this is for augmenting yourself." Because it is: the Maiden heals, the Warrior emboldens, the Mother strengthens, and the Crone hardens. The Four might help you, but nowhere in the lyrics does it suggest that they actually put in an appearance.
Personally, I'm expecting Ginny and Hermione to mention it to Doug, and when he reads it, he'll say, "Well no wonder you didn't summon anything: this is for augmenting yourself." Because it is: the Maiden heals, the Warrior emboldens, the Mother strengthens, and the Crone hardens. The Four might help you, but nowhere in the lyrics does it suggest that they actually put in an appearance.