Similes are an important part of the English language. We use them to convey meaning by comparing two dissimilar things and hoping that the listener will hear only the similarities. Often, though, people use similes without really understanding what they are doing.
An example of a good simile may be, "He ran away from the bee hive like a bat out of Hell." The word "like" suggests that we are going to compare two things. The things in this case are the fellow running and the bat in flight. We assume that bats flying out of Hell would probably be doing so with great haste. The simile is there to show that the man running away from the bee hive was also running quickly.
Then there are the examples of the bad similes. Take the sentence, "He watched television like a bat out of Hell." Again, we compare two ideas, watching television, and the bat in its hasty flight. It is extremely unlikely that the guy watched television wildly and with great speed. If someone said this sentence to you, you would be forced to look at that person "like" he was a moron.
I had a friend you used to use the phrase, "...like a banshee." For those unfamiliar, a banshee is a sort of ghost whose wailing would tell you that you or someone close to you would die soon. The sight of these creatures would be frightening, but the sound of the wailing would be worse.
It is acceptable to use the phrase, "like a banshee" when comparing unpleasant sounds. The defective washing machine belt screeched like a banshee. The cat stuck in the tree wailed like a banshee. Courtney Love sang like a banshee. It would probably be ok to say, "My mother in law keeps predicting people's deaths like a banshee."
Beyond those sorts of examples, the banshee simile loses something. It is not really right to say, "We went to the bar and we drank like a banshee." He ran away from the bee hive like a banshee. She sat there and knitted like a banshee.
That last example is based on something actually said. How does one knit like a banshee? What banshee like quality comes through in the act of knitting? I imagine the banshee sitting backstage, awaiting her performance. "Excuse me, Ms. Banshee, you're on in five." She would rise up from her knitting to say thanks, and then neatly tuck her yarn and needles away before going out to frighten some doomed traveler.
Of course, sometimes the wrong simile can be used for entertainment value. Some similes can have comedic value, like the proverbial spoon with a pig up its butt. When these "gaffs" are used intentionally and with skill, they aren't too bad. When they are used wrong because the speaker doesn't understand the concept of the simile, then you just have to feel sorry for that speaker.
Someday, through improvements in education and the regular beating of people who deserve it, blatant grammatical problems such as bad similes can be eradicated. They can be blown away like a school bus filled with the orphans of nuns.
You gotta pick the right guy to do the job.
Go out now and vote for LibertyBob.