“…difficult / challenging…” for whom ? There are dense poems which are very famous: “The Love Song of J. Alfred Prufrock” And “Dante’s Inferno” come to mind. An ice cream truck vendor once quoted Dante to me. There is more to the Times quote (They go on to discuss Alice Notley’s poetry), and it may apply more to very long poems, with dense language and references to classical literature which you may not have read, but, essentially, I am very much opposed to this statement (of philosophy). I believe that a good poem should be understandable and accessible to all. If you are writing for a tiny audience (e.g. university professors specializing in 17th century English literature) then ok, you can produce a poem which is difficult to read. But if you are writing poems for everyone who ever worked in an office and anyone who breathed the spring air, the poem should be crystal clear.