Franz Kafka wrote some rather good prose-poems. But I would rather agree that sometimes poetry can achieve a greater concentration and density of expression than prose. He was shot while resisting arrest, put in detention, then escaped and went into exile. Dennis Brutus attains a concentration and place which Alex Laguma has not achieved. The point is that if one is living in an abnormal society, then only abnormal expression can express that society.
Poetic commitment works in two ways.
As far as African poetry is concerned, especially up to the Second World War, up to 1945, you had people like David Diop whose poetry derives its strength from the force of the message. So the message becomes the poem, and if you apply the usual literary criticism, you will end up saying that this is just a political platform and not articulate poetry. Now that is not so. The extent to which one has suffered through political oppression is not necessarily the substance of a poem. One has to use certain techniques, certain concentrated, even visionary apprehensions of reality in order to convince the reader that the suffering is unique and meaningful and at the same time, a universal expression of life.