If you notice, much philosophy - generally defined, including religion - is vague. Much great art is vague. It may turn out that, unlike technical manuals, there is no value in spelling out everything. Either the reader engages or not and if they do, they don't need it spelled out - they create it in their own neural pathways, in their own mind, which is far more valuable than reading it. And if they don't, it doesn't matter anyway. It's not a new idea:
From the Christian Gospels (Mark 4)
Jesus tells the parable of the sower, ending, ""He who has ears, let him hear." And when he was alone, those around him with the twelve asked him about the parables. And he said to them, "To you has been given the secret of the kingdom of God, but for those outside everything is in parables, so that 'they may indeed see but not perceive, / and may indeed hear but not understand, / lest they should turn and be forgiven.'" And he said to them, "Do you not understand this parable? How then will you understand all the parables? The sower sows the word. And these are the ones along the path, where the word is sown: when they hear, Satan immediately comes and takes away the word that is sown in them. And these are the ones sown on rocky ground: the ones who, when they hear the word, immediately receive it with joy. And they have no root in themselves, but endure for a while; then, when tribulation or persecution arises on account of the word, immediately they fall away. And others are the ones sown among thorns. They are those who hear the word, but the cares of the world and the deceitfulness of riches and the desires for other things enter in and choke the word, and it proves unfruitful. But those that were sown on the good soil are the ones who hear the word and accept it and bear fruit, thirtyfold and sixtyfold and a hundredfold.""
From the Quran:
As for the unbelievers, alike it is to them / whether thou hast warned them or hast not warned them, / they do not believe. / God has set a seal on their hearts and on their hearing, / and on their eyes is a covering, / and there awaits them a mighty chastisement.
Regarding allegory in Medieval philosophy:
http://plato.stanford.edu/entries/medieval-literary/
The controversial and difficult question is why these medieval thinkers chose the allegorical form, and whether the text can be understood without its allegorical form. Avicenna tells us that what he purports to do by allegory is to convey one message to the "many" in sensible imagery they can understand, while conveying a different message to the philosophically minded few .... Neoplatonic and Christian writers ... [cited] the importance of not 'casting one's pearls before swine' ...