1) The writer
From the writer’s point of view, the reason for mixing metaphors is usually fear and laziness, a toxic cocktail.
The fear is that whatever he is writing is not interesting enough in and of itself. It is not a murder mystery, but a friggin’ merger of two banks, for instance. Or a wobble in the stockmarket. Or something else that Truman Capote never chose to write about. So, out of insecurity, he and his editor dramatize. They do that by aggrandizing the thing in question with words from more primal situations. Mergers become either “takeover battles” or “marriages.” Divestitures become “divorces”. Usually, war, love, sex, floods, fires, mountains and geology (“erosion,” “tectonic shifts,” “rifts”) find their way into the passage. And so on.
The laziness consists of not even noticing. They stuff these templates of primal experience into their paragraphs and don’t bother to think about what the words actually mean.
2) The reader
When the reader sees the outcome, he has one of two reactions. If sophisticated, he will notice the mixed metaphors and lose respect for the writer, usually in excess of what is justified, and probably stop reading the article, or at least taking it seriously.
If less sophisticated or hurried, he may not notice the mixed metaphor per se. But something more insidious now occurs. The different metaphors (floods, fires, quakes, wars) will cancel one another out in his mind. Instead of evoking strong and specific images, which is what metaphors are supposed to do, they produce a verbal goo. Its effect is tedium. The text loses energy and the reader gets tired and bored. The subject that the writer feared was not sufficiently interesting is now even less interesting. It’s excruciating.
3) The solution
In 99% of all cases, the solution is for the writer to address first the laziness (because that’s easier) and then the fear. To stop being lazy, just get in the habit of loving words and seeing their original meanings. To stop being afraid, get in the habit of forgetting about your audience entirely. You must find your subject interesting (otherwise, why not choose something else to write about?), and so it simply is interesting.
At this point, you’re ready to take all metaphors out of your text and see what’s left. Usually, it will be much better. The big secret about metaphors is that you don’t usually need them at all! Other details, from direct observation, take over.
Then, if you really feel something is missing, choose something evocative–but just one single image for the entire article–and stick with that. It doesn’t need to be “literary”. I heard a Congressman complain about the “bailout” package last week by saying that it was “a giant cow patty with a little marshmellow in the middle of it”. He didn’t even need to spell out that he was not going to swallow the marshmellow (by voting Yes), given what it was served in. Now that’s an effective metaphor, don’t you think?