In A Defence of Poetry Shelley makes a couple of statements that might engender debate. The first is that the Romantic poets, the poets of his present "memorable" age, were the best to come along since the Civil War and Cromwell's Interregnum. Poets of the succeeding Enlightenment Period (also called Neoclassical Period) are represented by names like Dryden, Pope, Samuel Johnson, Burns, Gray and Cowper. Since Shelley limits his observation to the period following the Interregnum, there is much to be said on his side of the argument, although there are those who might hold that Dryden, Pope and even Cowper had power not realized by the Romantics.
The second statement is that poetry is, and has always been, the greatest force behind any call to make a "beneficial change in opinion or institution." Two of the most world-changing events in history, the American Revolution and the French Revolution, were heralded and championed by prose written by such as Voltaire, Locke and Thomas Paine. Perhaps these two instances are exceptions to ancient trends, but based upon these two instances, one might argue against Shelley's contention that poetry is the "most unfailing herald ... awakening ... a great people to work a beneficial change."
The third statement is of interest in the contemporary debate about whether art, which of course includes poetry, merely reflects society or leads society. Shelley says, "Poets are the unacknowledged legislators [law makers] of the world." By this logic, the conclusion has to be that art leads society, governing the direction society will take, which puts the onus of responsibility for the shape of the world squarely upon the shoulders of poets and other artists.