One of the biggest problems with cliches is that they can be really confusing to beginning students of the English language. The expressions "through thick and thin" and "to lay it on a bit thick," for example, have to be explained to new speakers of English before they can grasp that the first expression means "through good times as well as bad times" and that the second expression means "being excessively generous with praise." Or consider the expression "as thick as pea soup." There is really no reason to suppose that a novice speaker of English has any idea what pea soup is like or how fog in any way resembles a hot green viscous vegetable-based liquid. Imagine the bewildering images suggested by a sentence like "The captain sailed his ship into a bank of fog as thick as pea soup." And what, pray tell, is a beginner to make out of a sentence like "Richard and Robert had been as thick as thieves in their youth"? I've been speaking English for over sixty years and I as yet have no idea at all what a "thick thief" is or how the expression ever came to mean "joined together by a deep and personal friendship." Perhaps I'm a bit thick, but it seems clear to me that the use of such cliches is a sure way to confuse new speakers of the English language.