Rock and Roll/Poetry

At what point to rock and roll lyrics become "poetry?" Or is there a difference between the two? I'm thinking of bands like They Might Be Giants, some of whose songs' lyrics often read like surrealist poetry (though their novelty hits are much more juvenile). Or even The Decemberists; Colin Meloy's lyrics are almost like narrative poetry. Why do rock and roll stars want to be poets, and why do some poets want to be rock and roll stars?