Social Commentary in Prog-Rock

The Great Debate by Dream Theater is another good one

Featured in RPI but unfortunately with lyrics in Italian: Fabrizio De Andre' is the best example. He was a sort of poet and it's a pity that his lyrics can't have the same strength when translated into English.

Even what is apparently a mainstream love song: "La Canzone di Marinella" (Marinella's song) Is about a prostitute found killed into a canal. 

He sang about war years before Roger Waters, his anti-religious invective is in the concept album "La Buona Novella", released before Aqualung. He sang about the French rebellion of the '68 in "Storia Di Un Impiegato e Di Un Non So", another concept album is "Non Al Denaro, Non All'Amore Ne' Al Cielo", based on Edgar Lee Masters book "Anthology Of Spoon River". There are songs like "River Sand Creek" about the Massacree of Native Americans. So I suggest grabbing a translation and listen to the albums. The two live albums with PFM added good prog arrangements to his singer-songwriter material.

Lots of songs  here and there but 2 that come quickly to mind are 21st Century Schizoid Man and Epitaph from KC's  first LP....

The Old Road is probably one of my favorite albums of all time. 

Actually, there is metaphor and allegory throughout The Court of the Crimson King, but the poem by Peter Sinfield is about Holy Roman Emperor Frederick II and his long antagonistic relationship with Popes Innocent III and Gregory IX, interspersed with allusions to Frederick's own writings regarding alchemy, religion and his acquaintance with Thomas Aquinas, along with other 14th century concerns.

For instance, the line "The Gardener plants an evergreen whilst trampling on a flower" is a reference to St. Francis of Assisi, the "Little Flower" whose ideals of charity and poverty were trampled by the pope and the Church. If you've ever read Umberto Eco's The Name of the Rose, the Spiritual Franciscans who advocated poverty as a Christlike ideal were supported by Emperor Frederick against the papal forces who were far more interested in the wealth and power of the Church in the 14th century.