Even the most plodding, sing-song, overwritten 19th century poem, bearing its heavy burden of classical allusions contains some thrilling lines that would make a great reading if they were set free. I admit to making a pretty good one page modern poem from Emerson's Threnody, the l-o-n-g poem he wrote on the death of his son. Of course, I edit and revise the Word of God as represented by Moses, David, Jesus and Paul. Everybody does that. But Emerson? That takes nerve, I admit, but not as much as editing Mary Oliver, and I have done that as well.
We do not quote people in readings because we seek their authority to prove our sermonic points. The authority of what we say rests on our personal authority and on the authority of the free pulpit (our listeners know that we gain no reward, nor suffer any penalty, for speaking the truth as we see it.)
Readings are a part of the worship service, a ritual in which the worshippers enter into an altered state of consciousness: softened by beauty and silence, turned back into themselves by their passive role, freed from the responsibilities of performance, finally unguarded enough to be receptive to invitation to respond to their own lives in a different, more trusting way.