Many of my favorite poems are devoid of metaphor. It is seldom used in haiku, for example. It would swamp them and overwhelm their simplicity. But nearly all poems make associations. Metaphor is simply a way to indicate likeness. The spectrum from identity through association to differentiation applies. In an engram, any datum can equal any datum, represented often as A=A=A. The dog is the man is the car is the tree is the sky is the smell of gasoline is the pain of a concussion is the voice of the man yelling something, etc. Along that spectrum, one could place metaphor, simile, allegory, symbol, juxtaposition and other devices that indicate some sort of likeness or degree of identity. In a way a simile is closer to differentiation than a metaphor, since it explicitly says that two things are similar, not the same.
In haiku, usually juxtaposition is used rather than metaphor. More is left for the reader to contribute. In some cases the juxtaposition is mainly between stillness and motion or what's perceived and a sense of the perceiver. That's pretty obvious in the poem usually considered the first haiku (old pond, frog jumps in, splash). I use no metaphor in the following:
It's even sadder than you think:
They were ALL good people.
I think it's one of my best poems. You may not care for it, of course. The poem does use devices, chief among them irony. And a vast omission that only becomes clear on rereading.
Use of metaphor is a two-edged sword: It allows compactness, multiple meanings interrelated with minimal baggage. It also invites reactivity. As Dr. Szasz points out, most of the illogic (and insanity) of psychiatry stems from the failure of psychiatrists to understand that "mental illness" is a metaphor, a way of saying that certain conditions are similar to diseases (illnesses). When one forgets that it is merely a metaphor, one gets in trouble. Similarly, Bush doesn't understand that "War on Terror" is a metaphor. He actually believes one can declare war on a level of the tone scale or an idea. Much metaphor-fraught poetry is similarly reactive. Sometimes the reactivity is the subject (getting us into a reactive viewpoint to gain insight), but just as often, the poetry itself is reactive. Metaphor is also the language of bigotry: Jewish swine, Commie pigs, etc.
So to me a metaphor is a responsibility. It's not something that makes a poem a better poem. It's something that may improve a poem or may ruin a poem. It's just another tool to be used wisely or unwisely.
by Dean Blehert, posted with slight corrections by his wife Pam